Handle empty flag attributes, such as gravity=""

Without this, an empty string will be passed into
XmlUtils.convertValueToInt, which throws an
index out of range exception.

Change-Id: Ibd1a140e207653b205852087f33ccb87fbbb9bbc

tools/layoutlib/bridge/src/android/content/res/BridgeTypedArray.java

@@ -218,7 +218,7 @@ public final class BridgeTypedArray extends TypedArray {
             return defValue;
         }
 
-        if (s == null) {
+        if (s == null || s.length() == 0) {
             return defValue;
         }