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: Id16039722fd70d4e2e934a0988f13a768ce833ec