Fix crash in getParentContainerBounds

Before, before TaskFragment appeared, the getTopNonFinishingActivity may
be null, and throw exception for no bounds found.
Now, make sure we always have Task bounds stored.

Bug: 228885031
Test: atest WMJetpackUnitTests:SplitControllerTest
Test: atest WMJetpackUnitTests:TaskContainerTest
Change-Id: I3744813a85f24941563c8ff8f284b2e45396f906