Add NEED_AIDL_NDK_PLATFORM_BACKEND

The config variable is used to force the generation of the AIDL
ndk_platform backend which will eventually be removed in favor of the
ndk backend. The switch is needed as an escape hatch for some devices
whose BSP (outsourced and thus hardly modifiable) depends on the
ndk_backend libraries.

Bug: 161456198
Test: m
Change-Id: I62c995642b820419eaeedeee616c47aee1da251d