Documentation for KeyEvent.getRepeatCount()

The existing documentation suggests that ACTION_UP events will contain a
valid repeat count that is equal to the repeat count of the latest
ACTION_DOWN event. However, in practice, that is not the case.

Rather than changing the behaviour to follow the existing documentation,
change documentation to conform to the existing behaviour. The rationale
here is that changing the behaviour might mean that some apps would
start relying on getRepeatCount() being non-zero. But this would only
work on the latest Android release. So if an app is tested only on the
latest Android release, and not on the release before this behaviour was
changed, the app would be broken on the older releases of Android.

Test: CTS tests will be added for injection
Bug: 113341746
Change-Id: Ia53e0e606c0b3c64135ef00950dc8a19f62933ca