liblog: remove Rwlocks for logd_socket and pmsg_fd

These historically used atomics to manage their lifetime.  They were
unfortunately unsafe and later replace with a RwLock.  A lock is
also problematic as it is too heavy weight for the typical use case
and implies that logging is neither async nor fork safe.

This change returns us to using atomics with two key changes:
1) compare_exchange_strong() is used instead of atomic_exchange().
   The latter has a race condition where a separate thread could have
   read the atomic value into a register, while the thread performing
   the atomic_exchange closes that FD.  The new code only changes the
   FD in the atomic if it is uninitialized.
2) Using the fact that DGRAM sockets can have connect() called on them
   multiple times, it uses a single logd_socket for the duration of
   the program.

These sockets are thread/async/fork safely created and accessed.

The one caveat is __android_log_close(), which is intended only to be
used by zygote when it is single threaded and is therefore not thread
safe.  It will close this socket and reset the underlying variable,
such that the next log message will go through the above
initialization.

Bug: 65062446
Test: logging works, logging unit tests
Test: new unit test
Change-Id: Ia4dbf7479dbe50683d124558ab2f83bff53b8f5f