This follows the principle of least surprise: the default identity
doesn't change when a new identity is created.

GroupActivity needs to receive a similar treatment.

This will allow any activity to show the expiry warning and password
prompt, and to start and stop BriarService.

SplashScreenActivity, SetupActivity, ExpiryActivity and PasswordActivity
are special cases.

This partially fixes dev task #40. CreateIdentityActivity is also called
from AddContactActivity, but that's likely to change soon so I'm not
going to fix it now.