adb: extract syncmsg structs.

Make it so that we can get the sizeof a member of syncmsg without having
an instance of syncmsg or doing something awful along the lines of
sizeof(reinterpret_cast<syncmsg*>(nullptr)->status).

Bug: http://b/150827486
Test: m adb adbd
Change-Id: I4830e7f90033c7706ff52cdd8d13e9cf40c73628
(cherry picked from commit eddae929)