New copyright headers

[aomarks]

1. Updates all license headers to new concise SPDX style
2. Updates copyright statement holder from The Polymer Project Authors to Google LLC
3. Renamed LICENSE.txt to LICENSE because it is slightly more common (the GitHub default)
4. Trivial formatting fix to LICENSE files so that we exactly match the spdx.org text
5. Updates package.json author field to Google LLC

All existing years are preserved.

Exactly matches what we did in lit/lit#1630