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: I14a1c57bd06f1f2aa52491f779c7030d4de03547
Change-Id: I7b9055909b901843631314b9577a9e578a739d00