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/tests/test_cli.c

@@ -89,7 +89,7 @@ static void test_options(void) {
     assert(!strcmp(opts->serial, "0123456789abcdef"));
     assert(opts->show_touches);
     assert(opts->turn_screen_off);
-    assert(opts->prefer_text);
+    assert(opts->key_inject_mode == SC_KEY_INJECT_MODE_TEXT);
     assert(!strcmp(opts->window_title, "my device"));
     assert(opts->window_x == 100);
     assert(opts->window_y == -1);