Add DeviceAsWebcam to general handheld_system

DeviceAsWebcam is a new service that lets an Android Device be used as a
UVC webcam. The logic in DeviceAsWebcam is protected behind the vendor
property `ro.usb.uvc.enabled`. Vendors who support UVC will need to set
the property to `true` using something like
```
PRODUCT_VENDOR_PROPERTIES += \
    ro.usb.uvc.enabled?=true
```

Bug: 242344221
Test: Manually tested that the makefile is correctly inherited from.
Change-Id: I9b8444406f1c02fc8b76caee3e961016d9430a0a