Update Android.mk to support file_contexts.bin

This change supports external/libselinux changes to implement
PCRE formatted binary file_contexts and general_file_contexts.bin
files.

The $(intermediates) directory will contain the original text file
(that is no longer used on the device) with a .tmp extension as well
as the .bin file to aid analysis.

A CleanSpec.mk file is added to remove the old file_contexts file.

Change-Id: I75a781100082c23536f70ce3603f7de42408b5ba
Signed-off-by: Richard Haines <richard_c_haines@btinternet.com>