layers: Fix unsafe bool mix warnings (part 2)

Fixes the MSVS warnings regarding unsafe mix of VkBool32 and bool for the
device_limits, image, parameter_validation, and threading layers.
Converts all Boolean usage internal to the layers to bool/true/false.  The
VK_TRUE and VK_FALSE values continue to be used with the Vulkan API
components.

Change-Id: I612d3f26050fadcd77ffca4d8723a7c734613816
diff --git a/layers/threading.h b/layers/threading.h
index 0e23363..2116875 100644
--- a/layers/threading.h
+++ b/layers/threading.h
@@ -63,7 +63,7 @@
     VkDebugReportObjectTypeEXT objectType;
     std::unordered_map<T, object_use_data> uses;
     void startWrite(debug_report_data *report_data, T object) {
-        VkBool32 skipCall = VK_FALSE;
+        bool skipCall = false;
         loader_platform_thread_id tid = loader_platform_get_thread_id();
         loader_platform_thread_lock_mutex(&threadingLock);
         if (uses.find(object) == uses.end()) {
@@ -146,7 +146,7 @@
     }
 
     void startRead(debug_report_data *report_data, T object) {
-        VkBool32 skipCall = VK_FALSE;
+        bool skipCall = false;
         loader_platform_thread_id tid = loader_platform_get_thread_id();
         loader_platform_thread_lock_mutex(&threadingLock);
         if (uses.find(object) == uses.end()) {