Fix LockSettingsService unit test flakiness

Flush the unlockUser() call in the handler thread before changing
the device credential again.

Bug: 131132243
Test: atest --generate-new-metrics 100 WeaverBasedSyntheticPasswordTests#testTokenBasedClearPassword
Change-Id: I1fdb5009caf744a7c1893c0e23d307bc9ee366c6
3 files changed