Let completeEnableRollback return a boolean (1/n)

Now #completeEnableRollback returns a boolean to indicate whether the
rollback object is successfully enable-completed or not.

Bug: 149258640
Test: atest RollbackTest StagedRollbackTest
Change-Id: Ib9803b4f6b12b3c31ea9a2d058f8be33150f0d51
1 file changed