Add build number

This adds the build number next to the pager dots in the new QS. It's a
standalone Composable, so it can be added in other places.

Test: manual, see that build number shows in correct conditions
Test: atest com.android.systemui.development
Flag: com.android.systemui.qs_ui_refactor_compose_fragment
Fixes: 378879498
Change-Id: Ie2db584cca5c936ac09c658187a1980dcae8c840