Fix AppOpsServiceTest flakiness.

AppOpsServiceTest creates multiple instances of AppOpsService, and
they write to the same file asynchronously, which may result in a
previous method overwriting the file being used by the current
method.

This change calls shutdown() after each test method, and removes any
pending write runnable from the handler. It also removes unnecessary
calls to readState() because state is always read in AppOpsService
constructor.

Fixes: 160785982
Test: atest AppOpsServiceTest
Change-Id: If792b61e5cae1d15601a18ef9cb0f71bc79d3d05