Don't add IME back and switcher buttons if IME renders them

- Also cache InputMethodService.canImeRenderGesturalNavButtons() since it doesn't change at runtime

Test: Enable persist.sys.ime.can_render_gestural_nav_buttons setprop, then ensure we don't draw IME back button or switcher button when gesture nav is enabled
Test: We still show back button in setup, regardless of nav mode
Bug: 205803355
Change-Id: Iab8c47521ab9e561881e4ffb45142888e4c427e4