Change default max digits of MetricStyle FixedFloat

Without knowing the measure/unit involved, 2 digits feels slightly better than 3, if only so that it's not confused for a locale-incorrect thousands separator.

Bug: 415827681
Test: atest NotificationMetricStyleTest
Flag: android.app.api_metric_style
Change-Id: Ie32931f07bd75fb2b330c9b4c13a100d4ec1f5e8