libcutils: Simplify android_get_control_socket()

1. TEMP_FAILURE_RETRY() isn't necessary with getsockname() because
it's a synchronous syscall.

2. There's no need to allocate and free a temporary buffer for the
full pathname of the socket.

Test: Manually, as follows:
    - added temporary ALOG output
    - flashed and booted a device
    - checked expected output from all system daemons
    - removed log statements

Change-Id: I4550a2d67fdffe6aff3c1050f8eeeaca0f985fb3