untrusted_app: allow access to /proc/tty/drivers

/proc/tty/drivers is read by applications to figure out if they are
running in an emulated environment. Specifically, they look for the
string "goldfish" within that file.

Arguably this is not an Android API, and really shouldn't be exposed to
applications, but:

1) A largish number of applications break if they can't read this file;
2) The information here isn't particularly sensitive

While we could spend a bunch of time trying to get applications fixed,
there are bigger fish to fry. It's not worth the battle.

Test: "ls -laZ /proc/tty/drivers" is labeled properly.
Bug: 33214085
Bug: 33814662
Bug: 33791054
Bug: 33211769
Bug: 26813932
Change-Id: Icc05bdc1c917547a6dca7d76636a1009369bde49