wifi(implementation): Add global lock

Add a global lock to address synchronization issues between the main
HIDL method servicing thread and the legacy HAL's event loop thread.

Also, added some documentation for the threading model used.

Bug: 34261034
Test: Compiles
Change-Id: I881111814ff5ebd601d6a4c85cf284b30ae47ed3
diff --git a/wifi/1.0/default/Android.mk b/wifi/1.0/default/Android.mk
index 144c067..ac484b6 100644
--- a/wifi/1.0/default/Android.mk
+++ b/wifi/1.0/default/Android.mk
@@ -19,6 +19,7 @@
 LOCAL_CPPFLAGS := -Wall -Werror -Wextra
 LOCAL_SRC_FILES := \
     hidl_struct_util.cpp \
+    hidl_sync_util.cpp \
     service.cpp \
     wifi.cpp \
     wifi_ap_iface.cpp \