Introduce input_method_navigation_bar_height

With the removal of the Taskbar InsetsSizeOverride for the IME in [1],
there would be a small discrepancy in large screen devices between
navigation modes. Three button navigation mode would use the taskbar
height (56dp), while gesture navigation mode would use the IME
Navigation bar height (48dp), which doesn't reflect the accurate size
between form factors.

This introduces a new dimension value for the IME navigation bar height,
matching the height of the system navigation bar in three button
navigation mode on non-large screen devices, and the height of the
taskbar in three button navigation mode on large screen devices.

  [1]: I5d40b7f99a197ca35d2db196711b0087aa146c74

Flag: EXEMPT bugfix
Bug: 407507279
Bug: 397328336
Test: manually verifying IME insets between navigation modes on
  non-large screen and large screen devices
Change-Id: I9a0cd7285d4d096ff5d9071d08ca5762947766f0