Change strings in MetricStyle to CharSequence

While still converting them to strings internally, dropping any and all spans. This leaves us the possibility of actually using rich formatting in the future, without breaking the API.

Bug: 415827681
Test: atest NotificationMetricStyleTest
Flag: android.app.api_metric_style
Change-Id: I9a06cce762d67396180c28df877cdc9cdd770405