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.

Ignore-AOSP-First: merge conflict expected when merged from AOSP.
Will cherry-pick after.

Bug: 161456198
Test: m
Change-Id: Iab535f4e2c9ba5017e9ee2e112ff20059e7f401b