Use Subtle (with fallback) for message encryption

Using the library that calls into tink.

Tested with both Subtle (treat-insecure-origin-as-secure) and PureJS (without treat-insecure-origin-as-secure) implementation and stepped through the code to make sure both code paths are exercised.

Test server: http://stansu1.kir.corp.google.com:9879/

PiperOrigin-RevId: 199854220