Add strings for fullbackup data erase confirmation

Added a new string called fullbackup_erase_dialog_message which
will be displayed when the user has seen the "opted into dolly"
text.
Old string remains in place for the case where the user
hasn't seen the text.
BUG: 20489775

Change-Id: Id6675d1510cf1598f84fdfdcda7f6502a6cf6715