libsnapshot: Improve vts_libsnapshot_test alternate configuration testing.

Change -force_config to -force_mode. Change inputs to vab-legacy
(Android R) and vabc-legacy (Android S). Default is device
configuration. There is no explicit option for Android T since it is the
default.

This also adds -compression_method which overrides the default of "gz".

Bug: 208944665
Test: vts_libsnapshot_test -compression_method lz4
      vts_libsnapshot_test -force_mode vab-legacy
      vts_libsnapshot_test -force_mode vabc-legacy
Change-Id: Iedc9461325229ccffd84478e1971f8b6b959e160