Fix NPE in DisplayContent.setTouchExcludeRegion.

DisplayContent.mTapDetector is null for any non-default display. When
testing ActivityView DisplayContent.setTouchExcludeRegion() throws an
NPE when dereferencing mTapDetector.

This change adds a null test to avoid the NPE.

Fixes bug 19413700.

Change-Id: I48a66c0ed071c87b38b354eead58a4de97fb47b9

services/core/java/com/android/server/wm/DisplayContent.java

@@ -241,7 +241,9 @@ class DisplayContent {
                 mTouchExcludeRegion.op(mTmpRect, Region.Op.DIFFERENCE);
             }
         }
-        mTapDetector.setTouchExcludeRegion(mTouchExcludeRegion);
+        if (mTapDetector != null) {
+            mTapDetector.setTouchExcludeRegion(mTouchExcludeRegion);
+        }
     }
 
     void switchUserStacks() {