Update PackageWatchdogTest

Make PackageWatchdogTest compatible to the changes that added
DeviceConfig flags to PackageWatchdog. This includes:
    * Make PackageWatchdog#setExplicitHealthCheckEnabled private and
    use DeviceConfig mechanism for changing that value instead
    * Disable TestLooper#startAutoDispatch
    * Other minor refinements that solve compatibility issues

Bug: 129335707
Test: atest com.android.server.PackageWatchdogTest
Merged-In: I7323dc65ec2957aeab128224864441bdf63c6f81

Change-Id: I7323dc65ec2957aeab128224864441bdf63c6f81