Ensure returned event is non-null

Currently, we aren't checking whether the returned event is null. This
causes NPE during a test run when the implementation is bad. The NPE
causes the entire test run to fail, rather than failing individual
tests. This is inconvenient when doing TDD.

In this change, add a matcher for downtime to simplify the code, and add
null checks for cases where a matcher wouldn't be convenient.

Bug: 211379801
Test: m inputflinger_tests && $ANDROID_HOST_OUT/nativetest64/inputflinger_tests/inputflinger_tests
Change-Id: Id0c3cf1908adee0d810c7e24f8507c14f689f999