[2/n] Refactor StartupLatencyLogger

1. Use kt stirng's $substitutions for StartupLatencyLogger
2. Add "otherwise =" for @VisibleForTesting

Bug: 278092752
Test: tested on phone, foldable and tablet, print latency logs in logcat
Change-Id: If56b90dafbff0e0b1f7ea076581d71a0b5271d71