Adds a property that allows applications to get the provisioning-unique serial number. Bug: 9175567 Also fixes some missing mutexes that were causing intermittent failures in calls to OEMCrypto due to concurrency issues. Bug: 9175583 Merge of https://widevine-internal-review.googlesource.com/#/c/5831/ from the Widevine CDM repository Change-Id: I1d7e3ca9f3b06da345022f5f0d64e0c17a5cedca