Allow hiding the IME navigation bar

This implements a mechanism to allow IMEs to request hiding the IME
navigation bar (containing the IME collapse and IME switcher buttons)
using existing InsetsControllers API.

To make this work, the previous insetsSizeOverride for the IME set on
the system navigation bar was removed, and the IME navigation bar is now
treated as a caption bar internally, providing insets from its own
process.

Test: atest
  InputMethodServiceTest#testRequestHideImeCaptionBar
  InputMethodServiceTest#testRequestHideThenShowImeCaptionBar
Bug: 292515843
Change-Id: I3d367ea4eec90e980853d7de29907020d24d2e11