Disable navbar contrast for ThemedNavBarKeyboard

With Ib8f4045b9e8e7bf65c13437133f114d91d0cb050 navbar always has
contrast thereby creating a solid navbar. However, for IMEs that wish to
draw a gradient, will need to have this disabled.
This change disables contrast for extended navbar gradient.

Fix: 130809016
Test: Manually verified:
    1. m -j ThemedNavBarKeyboard
    2. Install and enable
    3. Click "Extended dark nav bar" and verify gradient is visible.

Change-Id: I511bdcde0ef57c1484faace484db262b49fdfe16

samples/ThemedNavBarKeyboard/src/com/example/android/themednavbarkeyboard/ThemedNavBarKeyboard.java

@@ -56,6 +56,15 @@ public class ThemedNavBarKeyboard extends InputMethodService {
 
     private KeyboardLayoutView mLayout;
 
+    @Override
+    public void onCreate() {
+        super.onCreate();
+        if (BuildCompat.EFFECTIVE_SDK_INT > Build.VERSION_CODES.P) {
+            // Disable contrast for extended navbar gradient.
+            getWindow().getWindow().setNavigationBarContrastEnforced(false);
+        }
+    }
+
     @Override
     public View onCreateInputView() {
         mLayout = new KeyboardLayoutView(this, getWindow().getWindow());