Emulated external storage always has multi-user support using paths
like "/data/media/<user_id>". Creates and destroys these paths along
with user data. Uses new ensure_dir() to create directories while
always ensuring permissions.
Add external storage mount mode to zygote, supporting both single-
and multi-user devices. For example, devices with physical SD cards
are treated as single-user. Begin migrating to mount mode instead
of relying on sdcard_r GID to enforce READ_EXTERNAL_STORAGE.
Bug: 6925012
Change-Id: I9b872ded992cd078e2c013567d59f9f0032ec02b
5b1ada2562