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: I4f42b31fca67edb4d717142d71c0065765acdd97
Change-Id: I4f42b31fca67edb4d717142d71c0065765acdd97
(cherry picked from commit 2760fd1a19947cfb2e68a521b8d20af64c0014d2)
1 file changed