Improve Packagewatchdog performance

I38be25753e1be64c0f98649ba843bc09e28043d9 introduced a PackageWatchdog for system server. This cl improves the following:
1. Memory allocations
2. Lock invariants
3. Comments
4. Behavior if XML file gets corrupted

Test: Still builds
Bug: 120598832
Change-Id: I8a06761997ad5738d894504d3d11ac037cb99a82