Allow handling of all pending staged rollback sessions before rebooting

When there is an unattributable native service crash during boot, we
rollback all packages available for rollback. In the current
implementation, we reboot immediately when a staged rollback session
becomes ready, even though there can be other staged rollback sessions
which are still being handled.

In this CL, the logic is changed to allow the health observer wait for
all staged sessions to be handled before rebooting.

Bug: 141843321
Test: atest StagedRollbackTest#testNativeWatchdogTriggersRollbackForAll
Test: atest StagedRollbackTest
Change-Id: Ib2aaee22868e5502b68847aa7f64e428070f7e3c