Mock sourceDir for ActivityThreadClientTest

This just has to be non-null for the purposes of path
generation. The result is thrown away as the mocked LoadedApk
doesn't actually support updating its info, so this should have
no effect on the test logic.

Bug: 154807103

Test: atest android.app.activity.ActivityThreadClientTest

Change-Id: Id097a0f3be3c8822e679093347c9a0320fdba13e