Allow XML file paths to be customized with sysprop

Three properties are declared as vendor-init-settable:
ro.media.xml_variant.codecs
ro.media.xml_variant.codecs_performance
ro.media.xml_variant.profiles

media_codecs.xml can now be named
media_codecs${ro.media.xml_variant.codecs}.xml

media_codecs_performance.xml can now be named
media_codecs_performance${ro.media.xml_variant.codecs_performance}.xml

media_profiles_V1_0 can now be named
media_profiles${ro.media.xml_variant.profiles}.xml

Test: Rename "media_codecs.xml" to "media_codecs_test.xml",
set ro.media.xml_variant.codecs to "_test", then
call "stagefright -i".

Test: Rename "media_codecs_performance.xml" to
"media_codecs_performance_test.xml",
set ro.media.xml_variant.codecs_performance to "_test", then
run android.media.cts.VideoDecoderPerfTest.

Test: Rename "media_profiles_V1_0.xml" to "media_profiles_test.xml",
set ro.media.xml_variant.profiles to "_test", then
run vts_mediaProfiles_validate_test.

Bug: 142102953
Change-Id: I38fa2924e02363639d1cdc3dd85128e0652343ee