Remove canImeRenderGesturalNavButtons and sysprop

This method, and the corresponding system property
persist.sys.ime.can_render_gestural_nav_buttons were originally added
in [1], just before starting the implementation of gesture navigation
handling for IMEs. These were meant to serve as a feature flag,
initially turned off, and later permanently enabled.

The method to read it is hidden, and the sysprop itself does not have an
associated sepolicy. Based on the persist.sys group, this would
potentially be readable, but not writeable, outside of system. We never
actually write the value, and just use the default value when reading it
in the code, so this is essentially never set.

This removes the sysprop and the code path handling it being disabled,
which allows greatly simplifying the IME navigation bar implementation.

This was flagged recently to explicitly announce the removal, and now we
can remove the flag disallowDisablingImeNavigationBar.

  [1]: I7a2e9f08072a97d61b78dddb144282a5728d1407

Flag: EXEMPT flag removal
Bug: 281029564
Bug: 411106486
Test: atest InputMethodServiceTest#testShowHideImeNavigationBar_doesDrawImeNavBar
  InputMethodServiceTest#testShowHideImeNavigationBar_doesNotDrawImeNavBar
  InputMethodServiceTest#testBackButtonClick
  InputMethodServiceTest#testBackButtonLongClick
  InputMethodServiceTest#testImeSwitchButtonClick
  InputMethodServiceTest#testImeSwitchButtonLongClick
Change-Id: Iaddeb95af84a284f42b391eab3fa93e3caaa272f