Rename default keyboard implementation to "sdk"

Rename {keyboard,mouse}_inject to {keyboard,mouse}_sdk.

All implementations "inject" key events and mouse events, what differs
is the mechanism. For these implementations, the Android SDK API is used
to inject events.

Note that the input mode enum variants were already renamed
(SC_KEYBOARD_INPUT_MODE_SDK and SC_MOUSE_INPUT_MODE_SDK).

PR #4473 <https://github.com/Genymobile/scrcpy/pull/4473>