Enforce backup encryption in the UI, not just in effect

Encryption is required when using 'adb backup' on encrypted devices,
but the UI has not been enforcing this.  The failure mode is quite
confusing: the user can tap "back up my data" with no encryption
phrase supplied, and the confirmation activity disappears, but
the backup doesn't actually happen.

We now disallow confirming the backup unless a non-empty passphrase
has been entered into the UI.

In addition, tapping "do not back up my data" now finishes
the confirmation activity instead of just leaving it in a
confusing "dead" state.

Bug 19081192

Change-Id: I9ca69a08ff4322fe25129f9ca880b066c31525d0