Stash taskbar when IME is present, including during gestures

- SysUI removes SYSUI_STATE_IME_SHOWING when starting a gesture from an app, but because unstashing has implications on the gesture transition (e.g. clips the bottom of the app), we defer handling the ime hiding until the gesture settles. Repurposed the flow that swaps the taskbar background during the gesture to support this case as well.
- Delay the unstash when IME is closing, to align with the end of the IME exit transition
- Remove TaskbarViewController.ALPHA_INDEX_IME now that we stash when IME is opening, since stashing already hides the taskbar icons
- Also support passing a starting progress to the stashed handle reveal animation, to allow it to be reversed when cancelled. For example, when returning to an app that has IME showing, we first start unstashing because we're in an app, but then we get the signal that IME is attached so we stash again almost immediately (within a frame or two).

Test: In both 3 button and fully gestural, open a keyboard in an app, ensure taskbar gets out of the way and then reappears at the end when the keyboard is dismissed
Bug: 202511986
Change-Id: I93c298a98ba369ea6310466ff3f802231c582687