Merge from Widevine repo of http://go/wvgerrit/66865
This CL updates the comments for the code that uses
InstallKeyboxOrCert to install a keybox or oem cert at runtime. There
is also a new unit test for the device id. There are not many unit
tests because we don't want to accidentally overwrite the production
keybox or cert on a device when running tests.
bug: 111725154
test: unit tests
Change-Id: Ide9b3d4732f3f96a723a128907babe7e59c02ffc