Fix issue in CpuTimePerUid where microseconds are used as milliseconds

KernelUidCpuTimeReader reads microseconds, but they are written to a
millisecond field in atoms.proto. Introduced in ag/3693225.

See google3 change: cl/222044203

Test: build
Bug: 119853118
Change-Id: I170e996aa852e28a246a98226677907bb3560371