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: I87e6f6344fb95cb3277671a1a47d16cdcd2ba5ae
3 files changed