Propagate IME back disposition state to SystemUI

Previously the navigation bar and taskbar were incorrectly using the
back dispositon state as the source of truth for the IME visibility.
While this can only be set while the IME is visible, it can (and is)
also unset despite the IME still being visible. This was fixed in [1].

While the taskbar previously did not take into account the back
dispositon state, by the IME visibility changing, the rotation of the
back button would also change, leading to an equivalent state. However,
with the fix from [1], this would no longer happen, with one scenario
being showing the IME and then the IME swithcer menu, which temporarily
unsets the back disposition mode while the menu is visible.

This propagates the back disposition state to SystemUI, so taskbar can
respond to changes in this value, rather than in the IME visibility.

  [1]: Ic57cea49f5ff49132802083b4f0c9b0e82db1cf7

Flag: EXEMPT bugfix
Test: switch to 3 button nav, show IME, show IME Switcher menu,
  observe back button rotation
  atest NavigationBarTest#testSetImeWindowStatusSysuiState_ImeVisibleImeSwitcherButtonVisible
  NavigationBarTest#testSetImeWindowStatusSysuiState_ImeVisibleImeSwitcherButtonNotVisible
  NavigationBarTest#testSetImeWindowStatusSysuiState_ImeNotVisibleImeSwitcherButtonVisible
  NavigationBarTest#testSetImeWindowStatusSysuiState_ImeVisibleBackDispositionAdjustNothing
Bug: 366129400
Change-Id: I839faa97292fd8273db604177f2a51cc0dce1262