Create unique labels for /dev/snd/{seq,timer}

No core android component needs access to /dev/snd/{seq,timer}, but
currently audioserver, bootanim, init, system_server and ueventd have
access. Seq and timer have been the source of many bugs in the past
[1]. Giving these files new labels without explicitly granting access
removes access from audioserver, bootanim, and system_server.
Init and ueventd still require access for /dev setup.

TODO: Explore unsetting CONFIG_SND_TIMER device kernels.

[1] https://github.com/google/syzkaller/wiki/Found-Bugs

Test: media CTS "cts-tradefed run cts -m CtsMediaTestCases" on Bullhead
and Dragon completed with no denials.

Bug: 29045223
Change-Id: I2d069920e792ce8eef70c7b4a038b9e7000f39f5