Delete PanelViewController

Performed largely by automation, so I believe it to be far lower risk
than it may seem. The only manual parts were reducing protecteds to
privates, minor tweaks to the strange booleans IntelliJ needed to add
to TouchHandler.onTouch, and some minor style cleanups.

Bug: 242211739
Test: atest SystemUITests SystemUIGoogleTests
Change-Id: Ic77518871e6a0637320420aaa4cf457c2d5b7269