The server is currently a JAR, but it may ba an APK or a DEX, so the
variable name should not contain the type.
Rename the environment variable, the Meson options and the C
definitions.
Expose a 'prebuilt_server' option to pass the path of the prebuilt
binary, so that the build does not require Android SDK.
Usage:
meson builddir -Dprebuilt_server=/tmp/my_prebuilt_server.jar
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.
