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.

Makefile

@@ -34,7 +34,7 @@ build-portable:
 	[ -d "$(PORTABLE_BUILD_DIR)" ] || ( mkdir "$(PORTABLE_BUILD_DIR)" && \
 		meson "$(PORTABLE_BUILD_DIR)" \
 			--buildtype release --strip -Db_lto=true \
-			-Dapp:override_server_jar=scrcpy-server.jar )
+			-Doverride_server_jar=scrcpy-server.jar )
 	ninja -C "$(PORTABLE_BUILD_DIR)"
 
 release-portable: clean dist-portable-zip sums