Android.mk: drop polluting variables

Some of the ALL_*_FILES variables remained that were used
in a way that could not be cleared. Move them to lower
case variants and use a build recipe PRIVATE_*_FILES variable.
This avoids polluting the global namespace.

Change-Id: I83748dab48141af7d3f10ad27fc9319eaf90b970
Signed-off-by: William Roberts <william.c.roberts@intel.com>