Make getInetAddress generate less garbage.

This function is now on the critical path for URLConnection
since OkHttp calls it to figure out what the MTU for the
NetworkInterface is.

This function performs poorly when there are a large
number of interfaces available.

- Read /proc/net/if_inet6 exactly once, instead of
  once per interface.
- List the contents of /sys/class/net exactly once,
  and not once per interface.

Also adds a couple of tests for if_inet6 parsing.

TODO: Add a "zero copy" version of UnsafeByteSequence
and then move readIntFile over to that version.

bug: 11411129
Change-Id: Ieb1d5f0beaf462721f897a5f6376a4587f3aebaf