Initialize the app last used timestamps to 0 in test setup.

These tests are currently assuming the initial last used
timestamps to be 0 which is used to be true but not anymore
after the change Ib90b435b1efc34144f0461aaaf339fc6037ecdc2.
So, explicitly initialize the last used timestamps to 0
in the test setup to keep these tests working.

Bug: 225776572
Test: atest services/tests/servicestests/src/com/android/server/usage/AppStandbyControllerTests.java
Change-Id: I5a0246d1d9fec57bd37b941eb93701dfc7da7576