Remove ndk_platform backend. Use the ndk backend.

The ndk_platform backend will soon be deprecated because the ndk backend
can serve the same purpose. This is to eliminate the confusion about
having two variants (ndk and ndk_platform) for the same ndk backend.

Bug: 161456198
Test: m
Change-Id: I0b5b8e1191c40a6de17f1f8e46010a56dcb31976
Merged-In: I13f719d6911f678e494b5177641c515c72b49906
2 files changed