Assorted Key Session Code Cleanup

Merge from Widevine repo of http://go/wvgerrit/53202
and
Merge from Widevine repo of http://go/wvgerrit/53624

This change contains a variety of small tweaks to the
ContentKeySession and EntitlementKeySession classes that were
discovered while fixing b/78652567. There should be no change in
behavior from this patch. The fixes are:

1) Added missing headers and removed unnecessary headers.

2) Removed the unused keys_ member from EntitlementKeySession.

3) Renamed ContentKeySession's protected member function so that it is
not an overload of the public LoadKeys() function. This makes it
clearer what EntitlementKeySession::LoadKeys() is doing.

4) Added missing "virtual" and "OVERRIDE" keywords.

5) Added missing copyright headers.

6) Ran clang-format with Google style.

7) Correct missing OVERRIDE keywords.

Test: tested as part of http://go/ag/4674759

Change-Id: Icb0af886d7d3eb097b5dffbb716be6ac28f0916d

libwvdrmengine/cdm/core/include/key_session.h

@@ -1,3 +1,7 @@
+// Copyright 2018 Google LLC. All Rights Reserved. This file and proprietary
+// source code may only be used and distributed under the Widevine Master
+// License Agreement.
+
 #ifndef WVCDM_CORE_KEY_SESSSION_H_
 #define WVCDM_CORE_KEY_SESSSION_H_