Fix recent regression in DrawableContainer.

Ensure optical insets are never null.

Bug:11537082
Change-Id: Ib75be83909fac612ae947f501835285cd8a79862
diff --git a/graphics/java/android/graphics/drawable/DrawableContainer.java b/graphics/java/android/graphics/drawable/DrawableContainer.java
index 60e2376..af8441a 100644
--- a/graphics/java/android/graphics/drawable/DrawableContainer.java
+++ b/graphics/java/android/graphics/drawable/DrawableContainer.java
@@ -60,7 +60,7 @@
     private long mExitAnimationEnd;
     private Drawable mLastDrawable;
 
-    private Insets mInsets;
+    private Insets mInsets = Insets.NONE;
 
     // overrides from Drawable