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: I6481499a3471d8f1a5dde2dbd68ede98dffae35f
Change-Id: I6481499a3471d8f1a5dde2dbd68ede98dffae35f
(cherry picked from commit 1f6baebbba9bc3e80d727db79e9375801919d241)