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