Workaround the IME dismissing when dragging PiP.

- When the IME is shown, skip showing the dismiss scrim in the menu
  because starting the activity will cause a window focus change, which
  will close the IME (it will still be covered by the dismiss overlay).
  This is slightly inconsistent from dragging while the IME is not visible,
  but is preferable to having the IME close.
- Also, use the inset bounds to calculate the fraction, which aligns
  closer to the dismiss check (which is also based on the display bounds
  and not the current (possibly inset) movement bounds.

Bug: 37655837
Test: Drag PIP over IME, ensure that it does not hide the IME.
Change-Id: I86228371effce2d962ab4367497f5b1bf32456ec