Clarify key backup details when backup exists but is not trusted

[jryans]

When you sign in as a new device on an account that has key backup enabled already, but you don't trust it yet, we present some confusing text.

"Use key backup" as button text feels like it's telling me I haven't set it up
at all. We should change to "Connect this device ..." which seems more reassuring
and applies for both the "add to backup" and "restore from backup" use cases.

We should also update the backup status text for the case where a backup exists, but
it's not being used by the current device so that it's much more clear that you
have a backup at least.

These wording changes have already been made for other platforms:

• Clarify that use backup means restore matrix-org/matrix-react-sdk#2917
• Also say "Connect ..." on remaining key backup buttons matrix-org/matrix-react-sdk#2931
• Clarify key backup details when backup exists but is not trusted element-ios#2375