[google_apis] Move google_api_keys.cc to google_api_keys-inc.cc

File google/google_api_keys.cc is included directly from
google/google_api_keys_unittest.cc which is now forbidden by
PRESUMIT and adding any new test of this new file leads to a
presubmit error.

This CL simply moves the file to google_api_keys-inc.cc to respect
the new PRESUBMIT rules.

Note: Even if file google_api_keys-inc.cc looks like a new added file,
its contents are identical to the previous version of
google_api_keys.cc (it is a pure move of the previous file, without
any edits).

Bug: None

Change-Id: I992f890f40450456448465fa15b58ebc2478bed9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5426201
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Auto-Submit: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283110}