fix: use full screen intent properly

[naveensingh]

Type of change(s)

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

What changed and why

• Added missing android.permission.USE_FULL_SCREEN_INTENT permission and related startup checks. It's unclear why it's not added already when there are related strings for requesting this permission in the commons lib.
• Reworked when full-screen intent is used:
  • Incoming or outgoing call (locked): FSI
  • Incoming call (unlocked): manual activity start (no FSI) if user opted in to always use full-screen, otherwise FSI
  • Outgoing call (unlocked): manual activity start (no FSI)
• Formatted some code to improve readability

Tests performed

• Outgoing calls
• Incoming calls (locked device)
• Incoming calls (unlocked device)
• Incoming calls (unlocked device) with Always display incoming calls on full screen enabled

Closes the following issue(s)

• Closes Incoming call UI hidden by lock screen (must unlock to answer) #165

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.