InputDispatcher: rename Connection::monitor to ::isFocusMonitor

It's only used for focus monitors these days, and this name is clearer
when you see it in code.

Also take the opportunity to fix missing or unused #includes as well.

Bug: 245989146
Test: m checkinput
Flag: EXEMPT refactor
Change-Id: I45190104a9d4dd7f278253976db38d54174a3682