Fix bug in sl4a_client relating to starting apps. (#179)

diff --git a/mobly/controllers/android_device_lib/sl4a_client.py b/mobly/controllers/android_device_lib/sl4a_client.py
index 67469e4..2ef2238 100644
--- a/mobly/controllers/android_device_lib/sl4a_client.py
+++ b/mobly/controllers/android_device_lib/sl4a_client.py
@@ -57,7 +57,7 @@
 
     def check_app_installed(self):
         """Overrides superclass."""
-        out = self._adb('pm list package')
+        out = self._adb.shell('pm list package')
         if not self._grep('com.googlecode.android_scripting', out):
             raise jsonrpc_client_base.AppStartError(
                 '%s is not installed on %s' % (self.app_name,