Changed the swipe down action's start point in the mock game activity helper.

On some devices, the statusbar cannot be pulled down by swiping starting from the top
of activity in the test. An easy way to get rid of this problem is to use the
top screen's absolute coordinate as the start point instead of swiping from the top of activity.

Test: https://android-build.corp.google.com/abtd/run/L32800030002512366
Bug: 328972986
Change-Id: If0c213e4e561ae132c739fc709606d83dc48a556