Temporarily disable the new adb backup behavior

Fall back to using the legacy android:allowBackup flag for apps that
don't specify the new ALLOW_ADB_BACKUP property. This is done to not
break GTS tests that rely on adb backup since corresponding apps can't
use ALLOW_ADB_BACKUP yet (ag/13205188 is blocked with some failure that
I'm  currently investigating).

Bug: 175250895
Test: atest CtsBackupHostTestCases
Change-Id: Id40b2ae6b677afca18011077f57926518b28960f