LayoutLib: Allow '+' in float attributes.

Earlier change to fix the invalid conditional check inadvertedly
disallowed '+'.

Change-Id: I75a37926681fe3a496482d6fb75161ac75326109

tools/layoutlib/bridge/src/com/android/layoutlib/bridge/impl/ResourceHelper.java

@@ -379,7 +379,7 @@ public final class ResourceHelper {
         }
 
         // check the first character
-        if ((buf[0] < '0' || buf[0] > '9') && buf[0] != '.' && buf[0] != '-') {
+        if ((buf[0] < '0' || buf[0] > '9') && buf[0] != '.' && buf[0] != '-' && buf[0] != '+') {
             return false;
         }