Using SPDX license identifiers in copyright/license headers

[oddhack]

In https://github.com/google/amber/pull/897#discussion_r464720484 I asked whether it was possible for Google to use a more compact statement, e.g.

// Copyright <dates> The Amber Authors // SPDX-License-Identifier: Apache-2.0

rather than including the full license text in every file, and was pointed to the current policy at https://opensource.google/docs/releasing/preparing/#license-headers. So, this is to ask if this form of header could be added to the acceptable list. I understand the realities of making changes like this which necc. pull in corporate lawyers, and don't expect a quick answer, but think it's worth raising the question. I think this should be assigned to @cdibona who AIUI is the point person on open source licensing at Google.