[FirebaseCoreInternal] Sanitize app ID used in file system resource

[ncooke3]

Context

• The name of an app's heartbeats storage file includes the Firebase app ID. Since the Firebase app ID includes the ":" character, the filename also contains a ":".

  This works on Apple's latest filesystem but it's safer to replace each ":" with a "_"

• In practice, this code change will cause the below diff in an app's container:

  📂 Application Support/
  └── 📂 google-heartbeat-storage/
  -    └── 📄 heartbeats-1:123456789000:ios:abcdefghijklmnop
  +    └── 📄 heartbeats-1_123456789000_ios_abcdefghijklmnop

• Also, resolved outstanding TODOs in FirebaseCoreInternal.

#no-changelog

[paulb777]

Should there be a test for no colon's in the id?

Should this PR go into Firebase 9?

[paulb777]

Thanks!

[google-oss-bot]

Coverage Report ¹

Affected Products

• FirebaseFirestore-iOS-FirebaseFirestore.framework

  Overall coverage changed from 86.75% (916759e) to 86.79% (6c6486f) by +0.04%.

  Filename	Base (916759e)	Merge (6c6486f)	Diff
  exception.cc	23.68%	73.68%	+50.00%
  exception_apple.mm	89.66%	58.62%	-31.03%
  leveldb_key.cc	98.38%	98.60%	+0.22%
  write_stream.cc	94.37%	90.14%	-4.23%

Test Logs

• Base (916759e): https://github.com/firebase/firebase-ios-sdk/actions/runs/2199194565
• Merge (6c6486f): https://github.com/firebase/firebase-ios-sdk/actions/runs/2204137248

1. https://storage.googleapis.com/firebase-sdk-metric-reports/Jg9hFFhTPm.html