BackupManagerService#cancelBackups() releases mCurrentOpLock sooner

Incoming operations don't have to block until all backup ops to be
cancelled are handled.

Test: make gts -j40 && gts-tradefed run gts -m GtsGmscoreHostTestCases -t com.google.android.gts.backup.BackupManagerHostTest
Bug: 35767318
Change-Id: I1c6f7a1cfbfc18483311044a75e46cd4e8f9f4f3