[ Merge of http://go/wvgerrit/150630 ]
DeviceFiles uses a static variable for tracking license IDs which
have been reserved by a CDM session before officially storing the
license on the device. This variable was not protected by a mutex,
and a rare race condition would arise, either crashing the service
or getting it stuck in a loop.
This CL adds a mutex for protecting the set of reserved IDs.
Bug: 226555704
Test: device_files_unittest
Change-Id: Icdea88673c76c267b4b7db79697ec52ae8e2581e
a80a9fef26