Tear down agent after preflight rejection

We were failing to tear down a full-backup target process in several
error situations (preflight, quota exceeded, unforeseen), leaving the
app in an incoherent execution state for general operation.  Now we
properly tear down the target in all full-backup early exits.

Fix #63540605
Test: run cts -m CtsBackupTestCases -t android.backup.cts.BackupQuotaTest

Change-Id: Id8f6fe0381e85a8d8e4015fc6fd34bb840859e7a