Adding clean variables for some variables used in CTS.

1. A CTS package can specify the test runner used to launch
   its tests from the device. This is useful for preparing
   and cleaning the device through some shell commands if
   necessary.

bug:5946699

Change-Id: Ib1d6454c4274b1c37323235c9b3a195f3b9d8131
1 file changed