Add AvatarPicker to GSI

AvatarPicker is now moved to system_ext, but this is still required from
GSI as GSI keeps system_ext image within the system image. This change
adds AvatarPicker to Soong-defined GSI image.

Bug: 370352548
Test: N/A
Change-Id: Ib78ed5631b22ea30c0e59b14031541dc94922281