AndroidTarget: Updated default executables_directory

Now defaults to '/data/local/tmp' which is both executable and writable
on all android devices, including production ones.
diff --git a/devlib/target.py b/devlib/target.py
index 78697ca..da3d2af 100644
--- a/devlib/target.py
+++ b/devlib/target.py
@@ -880,7 +880,7 @@
             self.working_directory = '/data/local/tmp/devlib-target'
         self._file_transfer_cache = self.path.join(self.working_directory, '.file-cache')
         if self.executables_directory is None:
-            self.executables_directory = self.path.join(self.working_directory, 'bin')
+            self.executables_directory = '/data/local/tmp/bin'
 
     def _ensure_executables_directory_is_writable(self):
         matched = []