Avoid mutating requests HashSet from different threads.

Bug: 336780799
Test: verified manually,
1. Enroll fingerprint on affected device
2. Enable assistant and activating assistant on lockscreen
3. Enable AOD
4. Go to lockscreen
5. Long press the power button to trigger assistant on lockscreen
6. Say play the news
7. Press power button to go to AOD
8. Tap on the screen to go to lockscreen
9. Tap on the weather icon or calendar event in smart space or any notification on lockscreen
10. It should show the alternate bouncer with the Side FPS indicator
11. Authenticate with fingerprint
12. SFPS indicator should not linger after unlock.
Flag: NONE bugfix

Merged-In: Ied1e743db40ecffc0ed07aa506eaf9be82c57a0a
Change-Id: Iaee9ddb261977de39db43ce25befd5c4046671cb