Use enum for key injection mode

There was only two key injection modes:
 - the default one
 - the mode with --prefer-text enabled

To prepare the addition of another mode (--raw-key-events), use an enum
instead of a bool.

PR #2831 <https://github.com/Genymobile/scrcpy/pull/2831>

app/src/options.c

@@ -43,7 +43,7 @@ const struct scrcpy_options scrcpy_options_default = {
     .control = true,
     .display = true,
     .turn_screen_off = false,
-    .prefer_text = false,
+    .key_inject_mode = SC_KEY_INJECT_MODE_MIXED,
     .window_borderless = false,
     .mipmaps = true,
     .stay_awake = false,