Tap on task center in MultiDisplaySystemDecorationTests

The test taps on default display center to switch
focus to an activity. This lands between the two screens
of the DUT instead of the desired activity.

Solution:
As a fix tap on the task center.

Bug: 193433238
Test: run cts -m CtsWindowManagerDeviceTestCases -t
android.server.wm.MultiDisplaySystemDecorationTests
Verified on: Pixel 4 & development device

Change-Id: I7c36eaf1d00ba2c6fb5b9dc1315a66719c0a2456
(cherry picked from commit 0bee7261045623d05ccd05100bd12e66580c672e)
Merged-In: I7c36eaf1d00ba2c6fb5b9dc1315a66719c0a2456