libsnapshot: Fix test failures on certain configurations.

Due to how CF is built and tested, VABC is enabled even when not
supported by the kernel. To work around this add some logic in
libsnapshot and the test harness to recognize this situation and
silently flip off the VABC flag.

This also fixes the -force_mode option to vts_libsnapshot_test, so that
it will skip tests that aren't supported by the device.

Bug: 264279496
Test: vts_libsnapshot_test on 11-5.4 with vabc enabled
Change-Id: I4c1e88fac54732c21c88169a7e0dd664e3858b89