Prevent deadlock in RollbackManager#getAvailabeRollbacks

Throw an IllegalStateException if RollbackManager#getAvailabeRollbacks
is called from the handler thread.

Bug: 131745429
Test: manual test
        * Add a call to RollbackManager#getAvailabeRollbacks from
	RollbackPackageHealthObserver#onBootCompleted
	* Observe WTF logs and device bootlooping
Test: atest RollbackTest
Test: atest StagedRollbackTest
Change-Id: Icd649f0c9474ecf1e5468eb835d7cf4231f17fbc