Change USE_CLANG_PLATFORM_BUILD default to true.
Change USE_CLANG_PLATFORM_BUILD default to true.

When USE_CLANG_PLATFORM_BUILD is not set, default will be clang/llvm.
USE_CLANG_PLATFORM_BUILD=false can be used to select gcc as default.

BUG: 23163853
BUG: 26102335

Change-Id: I00604c2aef4849e8c3505b2c4002eb1c46cd1fd1

xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build