Replace meson subprojects by subdir

Since Meson 0.44, subproject_dir may not be '.' anymore. This implies we
must move app/ and server/ to a subprojects/ directory, which requires
to also change some gradle files.

Instead, just use subdir(), with options to disable building of the app
or the server.

app/meson.build

@@ -1,5 +1,3 @@
-project('scrcpy-app', 'c')
-
 src = [
     'src/main.c',
     'src/command.c',