ndk: Use __ANDROID__ instead of ANDROID.

The platform headers use the ANDROID macro as a platform test.
This macro is set by the NDK build script explicitely, but it
is better to use __ANDROID__, which is also set by our toolchain
as a compiler prebuilt macro.

This allows usage of these headers with our standalone toolchain.

Change-Id: I431c54e29e502bb9da7a52ef7a58a6341211b4db
3 files changed