Refactor use of __ANDROID__ macro

Refactor use of __ANDROID__ macro

We use the __ANDROID__ macro, which is provided by the toolchain, in
numerous places. This patch refactors the usage of this by defining a
new macro, ART_TARGET_ANDROID, that is being passed during build to
ART_TARGET_CFLAGS in Android.common_build.mk on the same line as
ART_TARGET. The codebase currently assumes that the existence of the
__ANDROID__ macro implies that we are compiling art for an android
target device. This is because, currently, target builds are compiled
with target toolchains that provide the macro, while host toolchains
do not.  With this change this assumption is still preserved. However,
in a future patch we will add the ability to compile art for a linux
target, and in that case the ART_TARGET_ANDROID macro won't be passed
anymore.

Change-Id:  I1f3a811aa735c87087d812da27fc6b08f01bad51