Set a default value for TARGET_BUILD_DENSITY

If no density value is specified when invoking tapas, we now export
a default value of 'alldpi'

Change-Id: I12b22a4cbd0ef604826aad66c5f7ab9271eb6d57

envsetup.sh

@@ -601,6 +601,9 @@ function tapas()
     if [ -z "$apps" ]; then
         apps=all
     fi
+    if [ -z "$density" ]; then
+        density=alldpi
+    fi
 
     export TARGET_PRODUCT=$product
     export TARGET_BUILD_VARIANT=$variant