MediaHiearchyManager: prevent NPE in `updateTargetState()`

For some reason, even after adding the checks for -1 in
`isCurrentlyInGuidedTransformation()`, there are still times when
`startHost` and/or `endHost` can be null, causing a NPE.

Fixes: 221754301
Test: N/A
Change-Id: I05d02ac7dcd6b0d8a850f9a8bbef8ce6205a8231