Fix adb backup for encrypted case

New behavior. Backup no longer uses the encryption password. This is in
part because that is hard with patterns, in part because it is a security
issue - the off line backup is much easier to brute force than the phone.

Instead, we simply insist on an encryption password if your device is encrypted
and locked.

Bug: 17159330
Change-Id: Ia22f84722522abf0b569a3ef1e16ead5527c726d