Adjust some behaviors on PermissionMonitorTest

- This is a test only change.
- Remove calling startMonitoring() in setup() because this method
  should be called only once, so each test needs to call
  startMonitoring() if the testing function is running on it.
- Return empty list instead of null when getting installed
  pacakges in setup(). This can help test to have default user
  MOCK_USER1 when they call startMonitoring().

Bug: 192431153
Test: atests FrameworksNetTests
Change-Id: Id2ffb056b378873c3ba6a8bb31b7dedb56ad6d46
1 file changed