Choose a better impossible value for mStatusBarLongPressDowntime

Lots of tests use 0L for the event time, so I'm switching to -1L,
which is equally impossible in production but more impossible in tests.

Test: ran the test with shade_expands_on_status_bar_long_press on
Bug: 371224114
Flag: com.android.systemui.shade_expands_on_status_bar_long_press
Change-Id: Id406aa056d2f8ccceb42aaaa3a93cb835acbc47e