Include NewRollbacks in result of getAvailableRollbacks.

This CL adds NewRollbacks to the return value of getAvailableRollbacks,
as long as the PackageRollbackInfo is complete (i.e. there is one for
each child session in the install).

Including these should avoid the potential race condition between the
install finishing and getAvailableRollbacks being called which affected
some of the tests. This means that the draining of the handler that was
previously added to getAvailableRollbacks can now be removed.

In order to avoid a similar race condition when rollbacks are expired, this
change also cancels corresponding NewRollbacks when expireRollbackForPackage
is called.

Bug: 136241838
Bug: 136548146
Test: atest RollbackTest
Test: manual local test with delay added to onFinished in the service callback
Change-Id: Ie4581d218077dc16e53c337a5bb76609cc50a57e
Merged-In: I015ee5925e38118c40f4b9e145f78fb12c0e2890