This commit replaces use_subzero with SWIFTSHADER_USE_SUBZERO and
unsupported_build with SWIFTSHADER_UNSUPPORTED_BUILD in Android.mk so
that it can be consistent with other variables.

Change-Id: Ia22cc88bc5804d7d2bea942bbceb2fc30c88aa18
Reviewed-on: https://swiftshader-review.googlesource.com/20588Tested-by: Logan Chien <loganchien@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>