Fix minor issues discovered with Multi-Display tests

- NPE when launching activity on secondary display
and there is a docked stack on primary.
- Activities not finished correctly on secondary display
when it is removed.

Change-Id: If527211e8192088790d32b9673b960535e27033f
Test: See ActivityManagerDisplayTests.
diff --git a/services/core/java/com/android/server/wm/DisplayContent.java b/services/core/java/com/android/server/wm/DisplayContent.java
index c8e35eb..e73acde 100644
--- a/services/core/java/com/android/server/wm/DisplayContent.java
+++ b/services/core/java/com/android/server/wm/DisplayContent.java
@@ -727,7 +727,8 @@
             win.getTouchableRegion(mTmpRegion);
             mTouchExcludeRegion.op(mTmpRegion, Region.Op.UNION);
         }
-        if (getDockedStackLocked() != null) {
+        // TODO(multi-display): Support docked stacks on secondary displays.
+        if (mDisplayId == DEFAULT_DISPLAY && getDockedStackLocked() != null) {
             mDividerControllerLocked.getTouchRegion(mTmpRect);
             mTmpRegion.set(mTmpRect);
             mTouchExcludeRegion.op(mTmpRegion, Op.UNION);