Add unit to FixedString metric values

FixedString can be used to represent quantities that cannot be expressed in a single number, and it makes sense for it to have a unit in those cases.

Bug: 415827681
Test: atest NotificationMetricStyleTest
Flag: android.app.api_metric_style
Change-Id: Idd733e64ab35481c5ed03309a67d74cef12f8665