Allow observers to be loaded correctly

Removes dependency on PackageManager when loading observers early
in the boot. This is done by associating each MonitoredPackage with
a "packageName" string rather than a VersionedPackage. Incoming
failure calls will be matched against this string.

Added test case to cover this condition.

Test: Manual. Install apk with rollback enabled,
      reboot, adb shell dumpsys rollback,
      ensure package is still being monitored
Test: atest StagedRollbackTest
Bug: 177793481
Change-Id: I282ac75a9e97a58c3c0bf3e8978fd010ec5294cd