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
Merged-In: Iab535f4e2c9ba5017e9ee2e112ff20059e7f401b
Change-Id: I7c09e3355df90b2a9ba0cc7c5ca4f716b8207f1d