Added a test for IMAP APPEND

It's a preliminary change for IMAP bug fixes.

- Fixed a potential bug in ImapFolder.setFlags where it'd throw
  StringIndexOutOfBoundsException if flags is empty.

- Added a generic flag to proguard.flags so that now all methods with
  the "ForTest" sufix are automatically preserved.
  Turned out it wasn't needed for this CL, but it should come in handy

Bug 2538076
Change-Id: I49a08afc196c7b7f1f30477dfc38ac5381045d84
1 file changed