remount: Treat disable verity error as fatal only if verity is enabled

If device doesn't enable AVB altogether, then it might not have a vbmeta
partition at all. In this case, we shall ignore disable-verity errors.
We still disable verity unconditionally to keep the logic simple, but we
ignore any disable-verity error if AVB is not enabled in the first
place.

Bug: 241688845
Test: adb-remount-test on emulator
Test: Test remount on yukawa
Change-Id: Ifc763b3f0ca6989550c139a8c3a2308c9c2a7c3e