Infer the activation state from the presence of a BackupManagerService.

The filesystem is checked when the BMS is created for a user so we
don't need to recheck it every time we perform a backup related
operation because wihout the BMS we shouldn't say the system is
read for the user to back up their data.

Change-Id: Iaaa7d7381182ba7e05173cc4f5d25ce61054e210
Test: atest CtsBackupTestCases CtsBackupHostTestCases TrampolineTest BackupManagerServiceTest
Fixes: 133584471