Monotonicity for NEED_AIDL_NDK_PLATFORM_BACKEND.

Hint to partners this will go away. We have limited
testing of this option, and we shouldn't maintain two
ways of handling NDK AIDL variants. So, this way
we can remove it in the future. :D

Bug: 161456198
Test: build
Change-Id: I5ae43b4f1b6c0d7253e82caeb338b69b7e00fdb6