Fix hardcoded "isDefault" setting

Bug:2661261

- Changed true to @bool/im_is_default
- isDefault was changed in https://android-git.corp.google.com/g/#patch,sidebyside,37627,3,res/xml/method.xml by mistake

Change-Id: Id1c20c292bed27523fb41f32514f2abdee11f992

java/res/xml/method.xml

@@ -22,5 +22,5 @@
 
 <input-method xmlns:android="http://schemas.android.com/apk/res/android"
         android:settingsActivity="com.android.inputmethod.latin.LatinIMESettings"
-        android:isDefault="true"
+        android:isDefault="@bool/im_is_default"
 />