Flag off disabling the IME navigation bar

In [1] we introduced canImeRenderGesturalNavButtons together with a
system property persist.sys.ime.can_render_gestural_nav_buttons to
enable toggling the IME navigation bar on/off. These were meants as a
feature flag, which was eventually permanently enabled, but not removed.

The method to read this value is hidden, and the sysprop itself does not
have an associdated sepolicy. Based on the prefix group it could
potentially be read, but never set (except of rooted devices). We never
actually set this value, and just resolve to a default value in the
code.

This initially flags off the code path that reads the sysprop and
potentially allows disabling it, to provide a strong signal that it will
soon be removed.

  [1]: I7a2e9f08072a97d61b78dddb144282a5728d1407

Flag: android.view.inputmethod.disallow_disabling_ime_navigation_bar
Bug: 402442590
Test: n/a
Change-Id: I9c64e1aa05583f5f7461e4e536ce9718a2e1bd13