More generic way to disable avmedia

Introduce a configure option --disable-avmedia. For non-desktop,
avmedia is always disabled (for now), and for desktop enabled by
default. Add AVMEDIA to BUILD_TYPE when enabled. Don't build avmedia
when disabled. Add HAVE_FEATURE_AVMEDIA to <config_features.h>. Bypass
avmedia-related code when disabled.

Change-Id: I919c6e199bff87d9f142d55dff6373f0ed217f39