Add aconfig flag for URI changes.

We currently have a classic flag for this (b/301242692), but we should
migrate it to aconfig.

Bug: 301242692
Bug: 281044385
Test: builds
Flag: ACONFIG com.android.app.visit_risky_uris DEVELOPMENT
Change-Id: Ia66e5b2c2b76e2b2be283c388f6f8f6b8d41d760