Wait for available rollbacks in RollbackTest.

This changes RollbackTest to retry checks for available rollbacks for
a certain period of time, using a new RollbackUtils method
waitForAvailableRollbacks.

Currently many of the tests in RollbackTest are flaky due to race conditions
between checking for available rollbacks and the installer onFinished callback.
This CL should fix all of those issues.

The previous attempt to resolve this by adding NewRollbacks speculatively to
the results of getAvailableRollbacks was not successful. This CL reverts that
change so that getAvailableRollbacks once again only returns rollbacks that are
actually available.

Bug: 136548146
Test: atest RollbackTest run on cuttlefish on acloud repeatedly
Change-Id: I0389ef2413e7642fe2f6ef1a33aef218dd1896ed
Merged-In: I27e2513b34f49af2f41cd627a5d2550541831e11