Merge from Widevine repo of http://go/wvgerrit/110024
The document now has a date for the R release, and talks about 16.4.
It also more clearly describes how the CDM matches the HIDL interface
version, not the Android OS version.
Test: Doc change only
Bug: 169155700
Change-Id: I463642fbb779388a2ff7758db1a03d998e89510c