Use stack center when tapping Anr dialog

In AnrTests
slowOnTouchEventHandleTriggersAnr and
embeddedWindowTriggersAnr launches an activity,
performs a tap on default display and waits for
the ANR dialog to appear,but in the multi display
area environment the activities under tests might
not be fullscreen.

Solution:
Instead of tapping on display center
we tap on the activity task center to make sure
we tap the desired activity.

Bug: 193545692
Test: run cts -m CtsWindowManagerDeviceTestCases -t
android.server.wm.AnrTests

Change-Id: Ife2057e401abf77276906f828dcdd1e6dd10002f
Merged-In: Ife2057e401abf77276906f828dcdd1e6dd10002f