adapt android jni to MSFactory usage, and add linphone_core_reload_ms_plugins() as a new way of loading plugins after LinphoneCore has been created.

This can be used on platforms where dynamic loading of plugins is not permitted, by doing things in this sequence:
1) Create the LinphoneCore as usual
2) call the libms*_init() routines of the plugins by passing them the MSFactory obtained via linphone_core_get_ms_factory()
3) call linphone_core_reload_ms_plugins()