Update eventTime to prevent stale events

Generate a new motionevent for each injection to ensure that eventtime
remains up to date.

In the InputDispatcher benchmarks, if the eventTime is not updated, then
the test will be stuck, because dispatcher will just drop all events.
To hit this condition, the benchmark must be ran for a sufficiently long
amount of time so that isStaleEvent returns true.
At that point, the benchmark for injectMotion will be stuck, because all
events are dropped, and we cannot consume anything.

The setDownTime api is unsupportedAppUsage in the Java layer, so we
shouldn't remove it.

Bug: none
Test: flame:/data/benchmarktest64/inputflinger_benchmarks #
./inputflinger_benchmarks --benchmark_min_time=10

Change-Id: I1eb93ca9ddac52562e0da50de355d5cdc584c893