Move virtual camera service (build flag)

from base_system.mk to handheld_system.mk.
Virtual camera service doesn't make sense on devices
without camera server (which is defined in handheld_system.mk).

Bug: 301023410
Bug: 311679099
Test: build
Ignore-AOSP-First: Virtual Camera HAL target is not merged in AOSP.
Change-Id: I67c18cfa7a2717cce738b17a53199fa595254068