adb: factor out socket specification.

Move the logic for string socket specification out to separate
functions to facilitate using arbitrary sockets for the adb command
socket.

Bug: http://b/30445394
Change-Id: Icd8fdb853272edc029fb3a0f5b18e941dc8ef52c
Test: adb_test, adbd_test, test_device.py
diff --git a/adb/Android.mk b/adb/Android.mk
index 3512323..1aa3dc1 100644
--- a/adb/Android.mk
+++ b/adb/Android.mk
@@ -52,6 +52,7 @@
     adb_utils.cpp \
     fdevent.cpp \
     sockets.cpp \
+    socket_spec.cpp \
     transport.cpp \
     transport_local.cpp \
     transport_usb.cpp \