fileutils: cache get_data_dir and other paths

Avoids to recompute data_dir repeatedly.
This improves performance on Android, avoiding repeated JNI calls
by many native threads.

jami_plugin_manager is moved to a unique_ptr
to delay creation to init(),
allowing to retrieve the proper data path.

Change-Id: I31236363dbc3eb03f9e59ac038729af6fb68ce0b