Prevent overwriting encryption keys

[jhaven-stytch]

Changes:

1. Change the encryption key property into a getter with a cached backing field, and lazy load it (Bug: Encryption Key is not present on relaunch #601). This helps reduce the chance that the key is fetched before protected data is available
2. If the key doesn't exist, ensure we don't try to read it if protected data is not available; instead print an error log and let the caller throw it's own error about the key not being available. This protects us from potentially overwriting the key if it was simply accessed before it was ready
3. Add a listener to the startup client to proactively fetch the encryption key when protected data becomes available to maintain first-access performance.

Checklist:

• I have verified that this change works in the relevant demo app, or N/A
• I have added or updated any tests relevant to this change, or N/A
• I have updated any relevant README files for this change, or N/A

[jhaven-stytch]

cc: @Sailor-Saturn, not sure if you got pinged on this, so just making sure it's on your radar!