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.

This removes the sysprop and the code path handling it being disabled.

  [1]: I7a2e9f08072a97d61b78dddb144282a5728d1407

Flag: EXEMPT refactor
Bug: 281029564
Bug: 411106486
Test: atest InputMethodServiceTest#testShowHideImeNavigationBar_doesDrawImeNavBar
  InputMethodServiceTest#testShowHideImeNavigationBar_doesNotDrawImeNavBar
  InputMethodServiceTest#testBackButtonClick
  InputMethodServiceTest#testBackButtonLongClick
  InputMethodServiceTest#testImeSwitchButtonClick
  InputMethodServiceTest#testImeSwitchButtonLongClick
Change-Id: I94e2fb280a1dac130596d7f4e344d83cfc12b70e

quickstep/src/com/android/launcher3/taskbar/NavbarButtonsViewController.java

@@ -72,7 +72,6 @@ import android.graphics.Region.Op;
 import android.graphics.drawable.Drawable;
 import android.graphics.drawable.PaintDrawable;
 import android.graphics.drawable.RotateDrawable;
-import android.inputmethodservice.InputMethodService;
 import android.os.Bundle;
 import android.os.Handler;
 import android.os.SystemProperties;
@@ -310,8 +309,7 @@ public class NavbarButtonsViewController implements TaskbarControllers.LoggableT
         }
         mNavButtonsView.setLayoutParams(navButtonsViewLayoutParams);
 
-        mIsImeRenderingNavButtons =
-                InputMethodService.canImeRenderGesturalNavButtons() && mContext.imeDrawsImeNavBar();
+        mIsImeRenderingNavButtons = mContext.imeDrawsImeNavBar();
         if (!mIsImeRenderingNavButtons) {
             // IME switcher
             final int switcherResId = Flags.imeSwitcherRevamp()