Add GIDs to packages.list, update SD card perms.

Write supplementary GIDs to packages.list for lower-level system
components to parse.

WRITE_EXTERNAL_STORAGE also implies sdcard_r GID. Switch to always
enforce READ_EXTERNAL_STORAGE permission. Update permission docs to
mention new behavior.

Change-Id: I316ba4b21beebb387ac05c80980ae9b38235b37d