utils/android: grant_app_permissions fix.

Handle the case where an app does not specify any permissions.
diff --git a/devlib/utils/android.py b/devlib/utils/android.py
index c5a79ad..170fda0 100644
--- a/devlib/utils/android.py
+++ b/devlib/utils/android.py
@@ -454,6 +454,8 @@
     permissions = re.search(
         'requested permissions:\s*(?P<permissions>(android.permission.+\s*)+)', dumpsys
     )
+    if permissions is None:
+        return
     permissions = permissions.group('permissions').replace(" ", "").splitlines()
 
     for permission in permissions: