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.

meson.build

@@ -1,6 +1,11 @@
-project('scrcpy', 'c', subproject_dir: '.')
+project('scrcpy', 'c')
 
-subproject('app')
-subproject('server')
+if get_option('build_app')
+    subdir('app')
+endif
+
+if get_option('build_server')
+    subdir('server')
+endif
 
 run_target('run', command: ['scripts/run-scrcpy.sh'])