Wrap start and stop user clients in an operation.

Addresses a race condition where newly queued operations can start before user switches complete.

Fix: 191798733
Test: atest UserAwareBiometricSchedulerTest
Test: manual (reboot -> Settings -> search Face & Fingerprint -> select Fingerprint -> menu opens successfully)
Change-Id: I425c84061a2aa5390d23e88d46155710988394b5