[SB Refactor] Make `numberOfLevels` a flow at the repo level.

The carrier merged logic will need to provide `numberOfLevels` at the
repo level, so pre-emptively refactor it there now.

Bug: 238425913
Test: `adb shell am broadcast -a com.android.systemui.demo
-e command network -e mobile show -e level 4 -e datatype 4g` -> shows
full triangle
Test: all statusbar.pipeline.mobile tests

Change-Id: I7f5bc07b4ff9917b32ffb6e56a1f18dbe181e401