Block getAvailableRollbacks on the handler thread.

To reduce flakiness in the CTS test, which checks for an available
rollback immediately after an app update is committed.

Remove sleeps from the RollbackTest that should no longer be needed.

Test: atest RollbackTest CtsRollbackManagerTestCases
Bug: 127933960
Change-Id: I60adb0f58eaba02d08ecd44b44866a8505c58238