standardize file header wrt licensing

We've been slightly inconsistent in the license header in files.
Standardize them so we can automate checking.

Change-Id: I3cdf85c9485d33cac2bb05c8080dfada3e5a5e8d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/500102
Reviewed-by: Gavin Mak <gavinmak@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>

git_trace2_event_log.py

@@ -1,3 +1,19 @@
+# Copyright (C) 2020 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Event logging in the git trace2 EVENT format."""
+
 from git_command import GetEventTargetPath
 from git_command import RepoSourceVersion
 from git_trace2_event_log_base import BaseEventLog