Generate stable MTP storage IDs.
Jeff Sharkey authored
It ends up that MediaProvider is persisting MTP storage IDs in its
database, so we need to make sure we generate stable IDs over time,
otherwise we can end up looking into a black hole.

Bug: 22256092
Change-Id: Ia419c37f87c7c0aa59ef0de530fd219c82988f6c
40fef25a