role::timedmediahandler: install libav-tools rather than ffmpeg

For the past several releases of Ubunutu, the 'ffmpeg' package has been a front
for the libav fork of ffmpeg. Trusty finally dropped the ffmpeg name
altogether; the package is now known libav-tools.

Bug: 67561
Change-Id: Ib90c94bd7e1ca9ae88a0103b9b08a8f309e752d4