AndroidTarget: Adds property to retrieve path of external storage

In andoid terms the external storage usually means the internal emulated
sdcard located at '/sdcard/'. This is the usually the root location used for
applications to read files that are not part of the application itself
from.

devlib/target.py

@@ -957,6 +957,11 @@ class AndroidTarget(Target):
         except KeyError:
             return None
 
+    @property
+    @memoized
+    def external_storage(self):
+        return self.execute('echo $EXTERNAL_STORAGE').strip()
+
     @property
     @memoized
     def screen_resolution(self):