Merge from master branch of Widevine repo of http://go/wvgerrit/66070
Merge from oemcrypto-v15 branch of Widevine repo of http://go/wvgerrit/63662
To make the threading model more clear, CopyBuffer is now a session function.
This means we need to pass in which session the current thread locks.
Test: unit tests.
Test: tested as part of http://go/ag/5501993
Bug: 113680369
Change-Id: I2fdd2cfcaab99f3793950b3845941463675f5e4c