Add native vsock support to ADB.

vsock is a socket address family for communicating into and out of
virtual machines. Addresses have a port and CID. The CID is unique to
each virtual machine on the computer. The VM host always has CID 2.
http://man7.org/linux/man-pages/man7/vsock.7.html

Inside the android guest, the adb daemon hosts a vsock server with
VMADDR_CID_ANY, automatically using the guest CID. The adb server
can now connect to addresses of the form vsock:cid:port, where the CID
must be specified and the port defaults to 5555.

This is a significant speed improvement for ADB connections in
Cuttlefish, with 150-200 MB/s for `adb push` and 100-150 MB/s for
`adb pull`. It also allows removing some proxying steps from Cuttlefish,
simplifying the full connection path, and removes a dependency on the
unstable ivshmem protocol.

Commands tested against a Cuttlefish VM with CID 3:
adb connect vsock:3:5555
adb -s vsock:3:5555 shell
adb disconnect vsock:3:5555

Supporting "adb disconnect" and "adb -s" required modifying some of the
parts that parse addresses / serials.

push/pull trials with native adb vsock support in cuttlefish:

100m: 1 file pushed. 167.9 MB/s (104857600 bytes in 0.596s)
100m: 1 file pushed. 171.1 MB/s (104857600 bytes in 0.584s)
100m: 1 file pushed. 175.2 MB/s (104857600 bytes in 0.571s)
100m: 1 file pushed. 168.7 MB/s (104857600 bytes in 0.593s)
100m: 1 file pushed. 172.9 MB/s (104857600 bytes in 0.578s)
100m: 1 file pushed. 168.8 MB/s (104857600 bytes in 0.592s)
100m: 1 file pushed. 197.5 MB/s (104857600 bytes in 0.506s)
100m: 1 file pushed. 201.0 MB/s (104857600 bytes in 0.497s)
100m: 1 file pushed. 199.0 MB/s (104857600 bytes in 0.503s)
100m: 1 file pushed. 215.5 MB/s (104857600 bytes in 0.464s)

/data/local/tmp/100m: 1 file pulled. 143.8 MB/s (104857600 bytes in 0.696s)
/data/local/tmp/100m: 1 file pulled. 137.5 MB/s (104857600 bytes in 0.727s)
/data/local/tmp/100m: 1 file pulled. 135.5 MB/s (104857600 bytes in 0.738s)
/data/local/tmp/100m: 1 file pulled. 139.6 MB/s (104857600 bytes in 0.716s)
/data/local/tmp/100m: 1 file pulled. 158.3 MB/s (104857600 bytes in 0.632s)
/data/local/tmp/100m: 1 file pulled. 126.7 MB/s (104857600 bytes in 0.789s)
/data/local/tmp/100m: 1 file pulled. 129.8 MB/s (104857600 bytes in 0.771s)
/data/local/tmp/100m: 1 file pulled. 154.8 MB/s (104857600 bytes in 0.646s)
/data/local/tmp/100m: 1 file pulled. 156.9 MB/s (104857600 bytes in 0.637s)
/data/local/tmp/100m: 1 file pulled. 152.2 MB/s (104857600 bytes in 0.657s)

Bug: 121166534
Change-Id: I50f21fb5c9acafb8daa789df4e28c9e1bbbbf2ef
Test: adb connect/shell/disconnect