Merge from Widevine repo of http://go/wvgerrit/127743
There was some confusion about who owned the OEMCrypto security level
string in a multithreaded environment. This is solved by caching the
security level at initialization time.
Bug: 188706160
Test: ran unit tests on bonito
Change-Id: I93af3bb2e5a8bf190627ee568f752b5ea9543306