[RefactoredBMS] Correct scope of "disabled?" check viz backup/restore decisions

This CL replicates ag/2204901 in RefactoredBMS.
Also adjusts tests and adds new ones for appIsDisabled method.

Test: runtest -p com.android.server.backup frameworks-services
Bug: 37973765
Change-Id: Ibaa15ba935ca1ada657e912b18a5a0b3bcffd00f