stagefright: list only the first codec for a given name

Now that we allow multiple codec stores and aliases, a codec name
may appear in multiple stores. List only the first codec for a
given name (with the lowest rank).

Allow this logic to be disabled for debugging via setprop
debug.stagefright.dedupe-codecs 0.

Bug: 119631295
Change-Id: I05f8ed86075a263e8becf80433018447f2f18361