Explicitly set generate_hashtree: false

Non-updatable APEXes are going to be only present on a dm-verity
protected read-only partition. For such APEXes there is no need in
embedding a hashtree.

This is a no-op change in preparation of switching default value of
generate_hashtree to true.

Bug: 190621617
Test: m
Change-Id: I426c1fce37ce63200810d69092660644e13291c2
Merged-In: I426c1fce37ce63200810d69092660644e13291c2
(cherry picked from commit 8f537e8762)

apex/Android.bp

@@ -58,6 +58,7 @@ apex {
         "bionic-linker-config",
     ],
     updatable: false,
+    generate_hashtree: false,
 }
 
 sdk {