Enable virtual external cameras (av)

Allow virtual external cameras with the mappedCameraId the same as
the actual camera id assigned by the HAL.

Bug: 375609768
Test: atest VirtualCameraTest
Flag: android.companion.virtualdevice.flags.external_virtual_cameras
Change-Id: Ifbac4db3215629c1be304b588612c8a82e2c5b7e