Set Gesture start time in UserActivityPokeTests

Set current time as start time on the first down event in
UserActivityPokeTests to simplify the test.

This is a followup from ag/31317516.

Test: atest inputflinger_tests
Bug: 245989146
Flag: EXEMPT TEST_ONLY
Change-Id: I87f9110ad84fef5950a76f4d02c178ff55ff870c