add simple text to recovery UI

- recovery takes a --locale argument, which will be passed by the main
  system

- the locale is saved in cache, in case the --locale argument is
  missing (eg, when recovery is started from fastboot)

- we include images that have prerendered text for many locales

- we split the background states into four (installing update,
  erasing, no command, error) so that appropriate text can be shown.

Change-Id: I731b8108e83d5ccc09a4aacfc1dbf7e86b397aaf