Imported from GitHub PR https://github.com/google/nearby/pull/2404
Copybara import of the project:
--
a6fbb3c396321c0b9012b787323f5f2ff652fd8f by Nick Bourdakos <bourdakos1@gmail.com>:
Add license header to all source files
Merging this change closes#2404
PiperOrigin-RevId: 623540004
Merged success/failure callbacks into a single callback
in SignInCallback and AccessTokenCallback to simplify resource management and to avoid making copies.
PiperOrigin-RevId: 565744310
Nick Bourdakos