Keep track of the WCT changes to determine the transition type

Before, we calculate the TransitionType based on the WCT, which may not
be accurate. For example, starting a new Activity to split may result
closing another TaskFragment.

Now, we keep track of where the WCT is started, and what exactly is
happening.

Bug: 207070762
Test: atest WMJetpackUnitTests:TransactionManagerTest
Change-Id: I19a21fac10fa47b75d696d9a5e273c1033ae3982