Added /system to search path

As documented at:
https://www.khronos.org/registry/OpenXR/specs/1.0/loader.html#android-active-runtime-location

Author: rbessems

src/common/platform_utils.hpp

@@ -302,7 +302,7 @@ static inline bool PlatformUtilsSetEnv(const char* /* name */, const char* /* va
 // Intended to be only used as a fallback on Android, with a more open, "native" technique used in most cases
 static inline bool PlatformGetGlobalRuntimeFileName(uint16_t major_version, std::string& file_name) {
     // Prefix for the runtime JSON file name
-    static const char* rt_dir_prefixes[] = {"/oem", "/vendor"};
+    static const char* rt_dir_prefixes[] = {"/oem", "/vendor", "/system"};
     static const std::string rt_filename = "/active_runtime.json";
     static const std::string subdir = "/etc/openxr/";
     for (const auto prefix : rt_dir_prefixes) {