Make sure timeout message is handled

This change waits for the looper to finish dispatching
timeout message to fix test flakiness.

Note that mTestLooper.moveTimeForward doesn't work
as ScanManager checks the system clock

Bug: 312001909
Bug: 312468159
Bug: 312418181
Flag: TEST_ONLY
Test: atest ScanManagerTest
Change-Id: I3e10161144ceeaea15b03cad3b4a8c4bb466704b