Set targetSdkVersion=31 to ThemedNavBarKeyboard

With this CL, ThemedNavBarKeyboard sameple IME will have an explicitly
set targetSdkVersion=31 so that it can be installed on older version
of Android OSes.

This CL also explicitly sets "android:exported", which has been
required in AndroidManifest.xml since API 31.

Fix: 211463997
Test: Manually verified as follows
 1. Flash SQ1A.211205.008 aosp_coral-userdebug into coral
 2. make -j ThemedNavBarKeyboard
 3. adb install -r $ANDROID_PRODUCT_OUT/system/app/ThemedNavBarKeyboard/ThemedNavBarKeyboard.apk
Change-Id: Idd88031a6b65cca58add8abe42794bc5f01b1973

samples/ThemedNavBarKeyboard/Android.bp

@@ -21,6 +21,8 @@ package {
 android_app {
     name: "ThemedNavBarKeyboard",
     srcs: ["**/*.java"],
+    min_sdk_version: "28",
+    target_sdk_version: "31",
     sdk_version: "current",
     dex_preopt: {
         enabled: false,

samples/ThemedNavBarKeyboard/AndroidManifest.xml

@@ -19,6 +19,7 @@
 
     <application android:label="ThemedNavBarKeyboard">
         <service android:name=".ThemedNavBarKeyboard"
+            android:exported="true"
             android:label="ThemedNavBarKeyboard"
             android:permission="android.permission.BIND_INPUT_METHOD">
             <intent-filter>

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

@@ -59,7 +59,7 @@ public class ThemedNavBarKeyboard extends InputMethodService {
     @Override
     public void onCreate() {
         super.onCreate();
-        if (BuildCompat.EFFECTIVE_SDK_INT > Build.VERSION_CODES.P) {
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
             // Disable contrast for extended navbar gradient.
             getWindow().getWindow().setNavigationBarContrastEnforced(false);
         }