Move android_get_control_socket out of line

android_get_control_socket has a warning from the implicit cast from
long to int.  The warning was being hidden because cutils/sockets.h was
included with -isystem.  Move android_get_control_socket to sockets.cpp,
since we don't want header only dependencies anyways, and fix the
warning with a range check and a static_cast.

Bug: 31492149
Test: m -j <module that uses sockets.h and -Wall>
Change-Id: I1f394ab26d4ec8a7dd0e7907c10416d7f8647624