AndroidTarget: Change android default working directory.

Changes the default working directory from
`/data/local/tmp/` to be on the devices external storage
(discovered from $EXTERNAL_STORAGE) which is usually `/sdcard`.
This is due to permission errors on some devices, to be readable
by android applications and will usually have a larger capacity.

Author: marcbonnici

devlib/target.py

@@ -1381,7 +1381,7 @@ def homescreen(self):
 
     def _resolve_paths(self):
         if self.working_directory is None:
-            self.working_directory = '/data/local/tmp/devlib-target'
+            self.working_directory = self.path.join(self.external_storage, 'devlib-target')
         self._file_transfer_cache = self.path.join(self.working_directory, '.file-cache')
         if self.executables_directory is None:
             self.executables_directory = '/data/local/tmp/bin'