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/keyboard_inject.h

@@ -18,7 +18,7 @@ struct sc_keyboard_inject {
     // number of repetitions. This variable keeps track of the count.
     unsigned repeat;
 
-    bool prefer_text;
+    enum sc_key_inject_mode key_inject_mode;
     bool forward_key_repeat;
 };