Fix broken SimpleSaveActivityTest#testTapUrlSpanOnCustomDescription_thenTapBack

Add FLAG_ACTIVITY_MATCH_EXTERNAL to make instant app can be resolved.

Bug: 148099813
Test: atest CtsAutoFillServiceTestCases:android.autofillservice.cts.\
SimpleSaveActivityTest#testTapUrlSpanOnCustomDescription_thenTapBack
Test: atest --instant CtsAutoFillServiceTestCases:android.autofillservice\
.cts.SimpleSaveActivityTest#testTapUrlSpanOnCustomDescription_thenTapBack

Change-Id: Idaf01fb61a5214eebcac3c9597e4f849123cf2eb