StrongPointer_test.cpp's Foo was colliding with RefBase_test.cpp's
Foo.
Test: out/host/linux-x86/nativetest64/libutils_tests/libutils_tests
from later CL
Change-Id: I2a4e956c88a07cec72d7ce734cf06c58134a4235
Arguably we should migrate to std::shared_ptr
but for now make std::vector<sp<>> a bit less
horrible
Change-Id: Ia458a2daff0b656b2f3310b3ea100565ec844c69