liblog: __android_log_pmsg_file_write() cleanup

__android_log_pmsg_file_write() will open /dev/pmsg0 if not
already, and will close it if we opened it.

Added atomic access to the android_log_context as insurance.

Fortify and correct pmsg tests.

Test: gTest liblog-unit-tests --gtest_filter=liblog.__android_log_pmsg_file_*
Bug: 31958686
Change-Id: I2cf6f971b6968938f471fda67367efe20dae3004
diff --git a/liblog/logger.h b/liblog/logger.h
index 8fb2b4d..d4f243a 100644
--- a/liblog/logger.h
+++ b/liblog/logger.h
@@ -17,6 +17,7 @@
 #ifndef _LIBLOG_LOGGER_H__
 #define _LIBLOG_LOGGER_H__
 
+#include <stdatomic.h>
 #include <stdbool.h>
 #include <log/uio.h>
 
@@ -31,9 +32,10 @@
 /* Union, sock or fd of zero is not allowed unless static initialized */
 union android_log_context {
   void *private;
-  int sock;
-  int fd;
+  atomic_int sock;
+  atomic_int fd;
   struct listnode *node;
+  atomic_uintptr_t atomic_pointer;
 };
 
 struct android_log_transport_write {