InputDispatcher: Inject to default display if display unspecified

Injecting a MotionEvent requires the target display to be known, because
the display projection must be applied to the event. If the displayId
for an injected MotionEvent is not specified, then inject to the default
display.

This is safe to do from a security standpoint because the permisson
check for input injection is based on the target of the input event, and
it happens later on in the dispatch cycle.

Bug: 179274888
Test: atest VerifyInputEventTest
Change-Id: I95970a46646fccfbfbc0f47b31d5c9df02254795