Fix logic in DeviceListener (#30230)

* remove incorrect check for cross-signing

SETUP_ENCRYPTION tries to set up everything (4S, cross-signing and key backup),
rather than just setting up encryption, as its name would imply.
crossSigningReady == false happens when the user's device isn't verified, so it
should trigger VERIFY_THIS_SESSION rather than SETUP_ENCRYPTION

* reorder conditions in allSystemsReady to match the order in the if statements

* explicitly handle secrets missing from 4S

rather than falling back to the SETUP_ENCRYPTION catch-all.  Also, remove
SETUP_ENCRYPTION since it is no longer used.

* convert button handlers to switch statements for consistency

(almost) all the other functions that use make decisions based on Kind use
switch statements

* update i18n (remove obsolete string)

src/i18n/strings/en_EN.json

@@ -969,7 +969,6 @@
         "reset_all_button": "Forgotten or lost all recovery methods? <a>Reset all</a>",
         "set_up_recovery": "Set up recovery",
         "set_up_recovery_toast_description": "Generate a recovery key that can be used to restore your encrypted message history in case you lose access to your devices.",
-        "set_up_toast_description": "Safeguard against losing access to encrypted messages & data",
         "set_up_toast_title": "Set up Secure Backup",
         "setup_secure_backup": {
             "explainer": "Back up your keys before signing out to avoid losing them."