Call startObservingHealth on handler thread

Both RescueParty and PackageWatchdog receive DeviceConfig callbacks.
RescueParty may act on this callback by calling startObservingHealth,
while PackageWatchdog will call updateConfigs. There may be a
contention for PackageWatchdog#mLock in this case. Move some of the
logic in startObservingHealth to the long task handler thread to avoid
this.

Bug: 150114865
Test: atest StagedRollbackTest
Test: atest RescuePartyTest
Change-Id: I74e1b21b7adddf35759e70211c47d31acfcae7f2