Validation/Default fallback Intent

Currently individual deep link handler methods/Activities need to do the following manually:

1. Validate the deep link URI. The framework takes care of most of this, but query parameters and parameter values still need to be validated manually.
2. Ensure certain pre-conditions are true (e.g. user must be logged in)
3. Be explicitly aware of the fallback Intent to use if the URI is invalid or a global pre-condition is false

It'd be nice if the framework could help reduce that boilerplate. Note a solution doesn't necessarily need to address all of the issues above. Let me know if this is something we want, and if so we can bounce on some ideas for what the API for this might look like.

The issue names no files or tests. Start by reading the individual deep link handler methods and Activities, then trace how URI validation, preconditions, and fallback Intents are currently handled. Done would require an agreed framework API and scope for reducing that boilerplate.

Written by the indexing model from the issue text.