Add ApiCompatibilityUtils#isInMultiWindowMode()

Based on MultiWindowUtils#isInMultiWindowMode() (which is in
org.chromium.chrome code, and therefore can't be used in the content layer),
but with simpler implementation.

We should probably update callers of MultiWindowUtils#isInMultiWindowMode() at
some point to use this method instead and then remove that method.

Bug: 
Change-Id: Ibb20fccf706368dd054541ce769855e1e24266ba
Reviewed-on: https://chromium-review.googlesource.com/578390Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Ryan Landay <rlanday@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488007}

base/android/java/src/org/chromium/base/ApiCompatibilityUtils.java

@@ -671,6 +671,17 @@ public class ApiCompatibilityUtils {
         }
     }
 
+    /**
+     * @param activity The {@link Activity} to check.
+     * @return Whether or not {@code activity} is currently in Android N+ multi-window mode.
+     */
+    public static boolean isInMultiWindowMode(Activity activity) {
+        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.N) {
+            return false;
+        }
+        return activity.isInMultiWindowMode();
+    }
+
     /**
      *  Null-safe equivalent of {@code a.equals(b)}.
      *