[SB] Add flag helper class for status_bar_mobile_icon_kairos.

Lets us more easily disable or enable the flag while using studio.

Bug: 413477995
Flag: com.android.systemui.status_bar_mobile_icon_kairos
Test: SysUI compiles
Change-Id: I375dcdba7d04dc4d0ee5bc45a7a5fec3c43edc55