Use "commit" instead of "execute" for rollbacks.

Which makes more sense for rollback of staged installs, where the caller
will have to reboot the device before the rollback takes effect.

Internal references to "execute" in IRollbackManager.aidl and the
rollback manager service implementation will be renamed in a follow up
CL.

Bug: 112431924
Test: atest RollbackTest
Change-Id: Ic2bb479873b309fc145e4eda3b347849ac219ac9