Better transition interruption and TextChange fixes

Previously, the TextChange transition didn't handle interruption/
cancellation at all, which made it problematic to use in any real
situation where a transition might get interrupted mid-animation.
Also, the way that it side-effected the text of TextView objects caused
errors in the UI when the transition was interrupted, because it would
not clean up after itself properly as new transitions queried the
current state of the UI.

Also, the prior cancellation logic for all transitions was not quite
correct; we were pausing transitions but resuming the animations, making it
tricky to write transitions that would restore state correctly.

Change-Id: I5a9f3c915e9834ec59ce1e1c3c96a88d11e4aa1b