Add Mobly test runner per SL4A_SL4A test script

To create SL4A_SL4A postsubmit pipeline, Mobly test runner is required per test script. (go/gd-sl4a-onboard)

Bug: 231518629
Change-Id: Id610b639730f4b8f107b3a036c9b4ca848e18774
Test: gd/cert/run