Legacy account migration

* Create new activity to encapsulate account upgrade
* Populate it with a list of legacy accounts, and progress bars for each
* Sidestep Welcome when there are legacy accounts to convert
* Super lightweight account migration:
  - Account login info only
  - no folders, messages, or attachments
* Scrub out old data
* Return to Welcome screen

As noted, the copies working (useable) POP & IMAP accounts, but does
not try to deal with folders, messages, or attachments.

Bug: 2065528
2 files changed