Improve the reliability of the SimpleSaveActivityTest

Call URLSpan.onClick from the UI thread.

bug: 155102708
Test: lunch hawk-userdebug && m
run cts --skip-device-info -s emulator-5554 -m CtsAutoFillServiceTestCases -t android.autofillservice.cts.SimpleSaveActivityTest#testTapUrlSpanOnSuccinctDescription_thenTapBack

Change-Id: Ie5afc25a462c7edb8a758ee7d5d44e889bfab4d7