Add explicit health check to PackageWatchdog

PackageWatchdog now uses the ExplicitHealthCheckController introduced
in Ia030671c99699bd8d8273f32a97a1d3b7b015d3b when observing packages.

Bug: 120598832
Test: Manually tested that after an APEX update, the network stack
does not pass the explicit health check until WiFi is connected
successfully. If Wi-Fi is never connected and the network stack
monitoring duration is exceeded, the update is rolled back.

Change-Id: I75d3cc909cabb4a4eb34df1d5022d1afc629dac3