Add glanceable_hub_v2 flag

The flag will be used to gate the refreshed hub experience in general,
which applies to both tablets and mobile.

Usages of the old flag will be cleaned up in a follow-up CL

Bug: 380122741
Bug: 375689917
Test: EXEMPT adding flag
Flag: com.android.systemui.glanceable_hub_v2
Change-Id: Ib1ac16d01864e3385a3d34ca80602581471f384c