-D__ANDROID_NDK__ for SDK builds.

The NDK is built like this. Having it for app builds within the Android
build system allows us to distinguish platform code from app code.

This is specifically to distinguish AIDL interfaces using libbinder_ndk.
'myBinder-ndk_platform' libs will use additional functionality that
'myBinder-ndk' libs won't use (in the future, these may be separate
variants with the same name, see b/121157555).

Bug: 136027762
Test: using this to distinuish code in NDK vs platform code
Change-Id: I27471b9184e8d67fc51c8eace317f92b3f0f3539