DO NOT MERGE: Add android.hardware.camera.external feature.

This feature will be listed by Android devices that allow for an
external camera to be connected to it. Such camera devices may
not always be available or connected.

Also clarify that android.hardware.camera.any also encompasses
devices that list android.hardware.camera.external.

Change-Id: I57497758877e4286c1f0116961731d483b56808f