fix: properly block MMS messages from unknown numbers

[naveensingh]

Type of change(s)

• Bug fix
• Feature / enhancement
• Infrastructure / tooling (CI, build, deps, tests)
• Documentation

What changed and why

Moved the unknown number check condition to the isAddressBlocked() function from mmslib. Without this, MMS messages from unknown numbers were always saved to the telephony database.

Tests performed

• I can't reliably test it.

Closes the following issue(s)

• Closes Add an option to block non-standerd "phone numbers" #610
• Closes 'Block messages from not stored contacts' does not block group chats from unknown numbers #189

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (if applicable).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• All checks are passing.