Add String16::c_str and start using it.

This is for parity with String8::c_str and in general the first step
in transition from String8|16.string() - to make it more similar to
std::string.

Bug: 295394788
Test: mma -j
Change-Id: I6c1411bef07c761fb2fb3fb38c27801ac4cffc57

libutils/String16.cpp

@@ -26,7 +26,7 @@ namespace android {
 
 static const StaticString16 emptyString(u"");
 static inline char16_t* getEmptyString() {
-    return const_cast<char16_t*>(emptyString.string());
+    return const_cast<char16_t*>(emptyString.c_str());
 }
 
 // ---------------------------------------------------------------------------
@@ -112,10 +112,7 @@ String16::String16(const char16_t* o) : mString(allocFromUTF16(o, strlen16(o)))
 
 String16::String16(const char16_t* o, size_t len) : mString(allocFromUTF16(o, len)) {}
 
-String16::String16(const String8& o)
-    : mString(allocFromUTF8(o.string(), o.size()))
-{
-}
+String16::String16(const String8& o) : mString(allocFromUTF8(o.c_str(), o.size())) {}
 
 String16::String16(const char* o)
     : mString(allocFromUTF8(o, strlen(o)))
@@ -173,7 +170,7 @@ status_t String16::setTo(const String16& other, size_t len, size_t begin)
         LOG_ALWAYS_FATAL("Not implemented");
     }
 
-    return setTo(other.string()+begin, len);
+    return setTo(other.c_str() + begin, len);
 }
 
 status_t String16::setTo(const char16_t* other)
@@ -200,7 +197,7 @@ status_t String16::setTo(const char16_t* other, size_t len)
 }
 
 status_t String16::append(const String16& other) {
-    return append(other.string(), other.size());
+    return append(other.c_str(), other.size());
 }
 
 status_t String16::append(const char16_t* chrs, size_t otherLen) {
@@ -286,7 +283,7 @@ bool String16::startsWith(const String16& prefix) const
 {
     const size_t ps = prefix.size();
     if (ps > size()) return false;
-    return strzcmp16(mString, ps, prefix.string(), ps) == 0;
+    return strzcmp16(mString, ps, prefix.c_str(), ps) == 0;
 }
 
 bool String16::startsWith(const char16_t* prefix) const

libutils/String8.cpp

@@ -150,10 +150,7 @@ String8::String8(const char* o, size_t len)
     }
 }
 
-String8::String8(const String16& o)
-    : mString(allocFromUTF16(o.string(), o.size()))
-{
-}
+String8::String8(const String16& o) : mString(allocFromUTF16(o.c_str(), o.size())) {}
 
 String8::String8(const char16_t* o)
     : mString(allocFromUTF16(o, strlen16(o)))
@@ -267,7 +264,7 @@ status_t String8::append(const String8& other)
         return OK;
     }
 
-    return real_append(other.string(), otherLen);
+    return real_append(other.c_str(), otherLen);
 }
 
 status_t String8::append(const char* other)

libutils/include/utils/String16.h

@@ -53,6 +53,7 @@ public:
 
                                 ~String16();
 
+    inline  const char16_t*     c_str() const;
     inline  const char16_t*     string() const;
 
 private:
@@ -234,6 +235,11 @@ inline int strictly_order_type(const String16& lhs, const String16& rhs)
     return compare_type(lhs, rhs) < 0;
 }
 
+inline const char16_t* String16::c_str() const
+{
+    return mString;
+}
+
 inline const char16_t* String16::string() const
 {
     return mString;
@@ -241,7 +247,7 @@ inline const char16_t* String16::string() const
 
 inline std::string String16::std_string(const String16& str)
 {
-    return std::string(String8(str).string());
+    return std::string(String8(str).c_str());
 }
 
 inline String16& String16::operator=(const String16& other)

libutils/include/utils/String8.h

@@ -193,14 +193,14 @@ public:
      * replaces whatever was there before.
      */
     String8& appendPath(const char* leaf);
-    String8& appendPath(const String8& leaf)  { return appendPath(leaf.string()); }
+    String8& appendPath(const String8& leaf) { return appendPath(leaf.c_str()); }
 
     /*
      * Like appendPath(), but does not affect this string.  Returns a new one instead.
      */
     String8 appendPathCopy(const char* leaf) const
                                              { String8 p(*this); p.appendPath(leaf); return p; }
-    String8 appendPathCopy(const String8& leaf) const { return appendPathCopy(leaf.string()); }
+    String8 appendPathCopy(const String8& leaf) const { return appendPathCopy(leaf.c_str()); }
 
     /*
      * Converts all separators in this string to /, the default path separator.
@@ -255,7 +255,7 @@ inline const char* String8::string() const
 
 inline std::string String8::std_string(const String8& str)
 {
-    return std::string(str.string());
+    return std::string(str.c_str());
 }
 
 inline size_t String8::size() const