Camera: Add user tag to camera action event

The user tag set by CaptureRequest.setTag() is passed to cameraservice.
The cameraservice saves the latest tag's string description into the
camera action event's session data.

Test: ./out/host/linux-x86/bin/statsd_testdrive 227
Bug: 223671217
Change-Id: I604d8b3bd3895c330de5b02498c7eb3ed8e05ed8
diff --git a/services/camera/libcameraservice/utils/SessionStatsBuilder.cpp b/services/camera/libcameraservice/utils/SessionStatsBuilder.cpp
index 7a7707c..c3aac72 100644
--- a/services/camera/libcameraservice/utils/SessionStatsBuilder.cpp
+++ b/services/camera/libcameraservice/utils/SessionStatsBuilder.cpp
@@ -60,6 +60,7 @@
     mErrorResultCount = 0;
     mCounterStopped = false;
     mDeviceError = false;
+    mUserTag.clear();
     for (auto& streamStats : mStatsMap) {
         StreamStats& streamStat = streamStats.second;
         streamStat.mRequestedFrameCount = 0;