Marking SystemApi for PackageWatchdog and Observer

We would be moving PackageWatchdog it's observers to new module.
All the api calls across boundary would need to be happen via SystemApi.
Marking the required APIs as SystemApi.

Bug: 361126781
Test: atest CtsRootPackageWatchdogTestCases
Flag: android.crashrecovery.flags.enable_crashrecovery
Change-Id: I40c15975d3e00958b265f9a1f8dd72cdea412902