seccomp: Fix policy for ffmpeg

Without this, a bunch of cts tests fail.  For example:

android.media.cts.AdaptivePlaybackTest#testMpeg4_adaptiveDrc

fails with the log message:

E/media.codec( 8194): libminijail: blocked syscall: getdents64

and

android.security.cts.StagefrightTest#testStagefright_bug_25765591

fails because of a blocked call to nanosleep.

Change-Id: Iba99163c86e2941a8e821136188ddb3cf4d34a5c