[Ongoing Call] Remove TODOs that are no longer relevant.

- system_gestures_start_threshold shouldn't ever change size.
- statusBarHeight does seem like the appropriate dimension to use to
determine the appropriate gesture area.

Bug: 195839150
Test: N/A (just removing code comments)
Change-Id: I1076d83120df0e1ea51d4e8d568a7a270cf35911