audio VTS: Make checks for supported methods more consistent

Fix the issue pointed out by gkaiser@ on aosp/2256190.
I also went over other usages the logic which checks for
methods support, and made the code more consistent.

Bug: 205884982
Test: atest VtsHalAudioCoreTargetTest
Change-Id: I1b06c1fd9dddcbdadfd6e04525493a049283f31f