Add some sleep time to allow grace period to expire.

BG-FGS while-in-use restriction uses the same 10 seconds grace period mechanism as BG-Activity-Launch,
starts from last activity's launch time or finish time.

Add 12 seconds sleep time to let the 10 seconds grace period expire so it won't
interfere with other BG-FGS while-in-use restriction features.

Bug: 182400165
Test: atest cts/tests/app/src/android/app/cts/ActivityManagerFgsBgStartTest.java
Merged-In: I5d9056feeac05574f73bab9a9c1b37cd6aa881dd
Change-Id: I4adc0759d36cc3669a43eaff2dedd6b50d308f00