Enable interacting with caption through status bar.

Adds logic to allow a drag down gesture to enter desktop mode from a
fullscreen task. This prevents status bar from opening notification
shade if the input is on handle. Also enables interacting with caption
menu buttons.

Bug: 247551213
Test: Drag handle in fullscreen task to enter desktop mode. Confirm
handle/caption menu buttons work through status bar. Confirm no
other caption functionality is affected.

Change-Id: I19bb9d750a9305939e4672429d9cce153290a014