Add Packagewatchdog to monitor health of apps on the device

Components within system_server can register with the PackageWatchdog, a list of packages to be monitored for excessive crashes over a given period. If any of those packages crashes excessively the component will be notifed of the crashes. Potential clients are RollbackManagerService and RescueParty.

Test: Manually tested with a modifed RescueParty registering and starting observation of a package
Bug: 120598832
Change-Id: I38be25753e1be64c0f98649ba843bc09e28043d9