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: I674244ff9b873d3779d3d649822389ad441fa0af
Change-Id: I612a89cb553f858a62f9e92e4bcd8455c6acab80