Split file_contexts for on-device compilation.

Simulate platform and non-platform split by compiling two different
file_contexts files and loading them together on-device.  Leave the existing
file_contexts.bin in place until we're ready to build images based on the new
files.

Bug: 31363362
Test: Builds and boots without additional denials.
Change-Id: I7248f876e2230cee3b3cbf386422063da1e3dde0

Bring back file_contexts.bin.

Change-Id: Ifec2c363579151080fdec48e8bc46bbbc8c97674
Signed-off-by: Sandeep Patil <sspatil@google.com>