Leanback: Disable 9-patch shadow on API-19

There is a double applied delta issue in optical-bounds
causing a negative measured size.

Bug was fixed in API-20 and after:
https://googleplex-android-review.git.corp.google.com/#/c/431321/

Bug 30943877

Change-Id: I82f48f3c2b73884854691a4a67673a08ebed0594
diff --git a/v17/leanback/src/android/support/v17/leanback/widget/StaticShadowHelper.java b/v17/leanback/src/android/support/v17/leanback/widget/StaticShadowHelper.java
index 022ff1d..aa82866 100644
--- a/v17/leanback/src/android/support/v17/leanback/widget/StaticShadowHelper.java
+++ b/v17/leanback/src/android/support/v17/leanback/widget/StaticShadowHelper.java
@@ -83,7 +83,7 @@
      * Returns the StaticShadowHelper.
      */
     private StaticShadowHelper() {
-        if (Build.VERSION.SDK_INT >= 18) {
+        if (Build.VERSION.SDK_INT >= 21) {
             mSupportsShadow = true;
             mImpl = new ShadowHelperJbmr2Impl();
         } else {