Build the BitmapShader if it hasn't been built yet.

This was causing a bug in the InCall UI where the background would be stretched.

Change-Id: Ie25a7d09f4779f166e267b2f921d40441ce361d2
diff --git a/graphics/java/android/graphics/drawable/BitmapDrawable.java b/graphics/java/android/graphics/drawable/BitmapDrawable.java
index e82f297..f42788e 100644
--- a/graphics/java/android/graphics/drawable/BitmapDrawable.java
+++ b/graphics/java/android/graphics/drawable/BitmapDrawable.java
@@ -271,7 +271,8 @@
 
     public void setTileModeXY(Shader.TileMode xmode, Shader.TileMode ymode) {
         final BitmapState state = mBitmapState;
-        if (state.mTileModeX != xmode || state.mTileModeY != ymode) {
+        if (state.mPaint.getShader() == null ||
+                state.mTileModeX != xmode || state.mTileModeY != ymode) {
             state.mTileModeX = xmode;
             state.mTileModeY = ymode;
             mRebuildShader = true;