Add the device config GenerateAidlNdkPlatformBackend

The config 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: Ie11ca62be05754bb2593da16e1380103c670b393