GpuService: secure setUpdatableDriverPath

setUpdatableDriverPath should only be called by system_server and
developer driver path needs to be protected by a lock.

Bug: 162383705
Bug: 159240322
Test: ./gapit validate_gpu_profiling --os android
Change-Id: I48896325598acab89079dbc658ddf9b92d303244
diff --git a/services/gpuservice/GpuService.h b/services/gpuservice/GpuService.h
index 43faa3e..409084b 100644
--- a/services/gpuservice/GpuService.h
+++ b/services/gpuservice/GpuService.h
@@ -79,7 +79,8 @@
     std::shared_ptr<GpuMem> mGpuMem;
     std::unique_ptr<GpuStats> mGpuStats;
     std::unique_ptr<GpuMemTracer> mGpuMemTracer;
-    std::string developerDriverPath;
+    std::mutex mLock;
+    std::string mDeveloperDriverPath;
 };
 
 } // namespace android