Include user identifier in external storage paths.

When building external storage paths, always include user in path
to enable cross-user paths and aid debugging.

Each Zygote process continues to only have access to the appropriate
user-specific emulated storage through bind mounts. A second set of
mounts continue supporting legacy /sdcard-style paths. For example,
a process running as owner has these mount points:

/storage/emulated_legacy
/storage/emulated_legacy/Android/obb
/storage/emulated/0
/storage/emulated/obb

Since Environment is created before Zygote forks, we need to update
its internal paths after each process launches.

Bug: 7131382
Change-Id: I6f8c6971f2a8edfb415c14cb4ed05ff97e587a21