[Android O] Refactor channel definitions into new class

[Android O] Refactor channel definitions into new class

- As discussed, we can give this a per-file OWNERS for added safety
in case of channels being updated without increasing the version number.

- In future there will also be a requirement to move deprecated channels
into a new map, and thus a further need for safety.

- This refactoring will also make testing easier as fake channel
definitions can now be easily supplied in tests.

Review-Url: https://codereview.chromium.org/2832433002
Cr-Commit-Position: refs/heads/master@{#466316}
(cherry picked from commit 8aacc8f8d02e5b19f491b34f744085dd2c521167)

BUG=710843
Review-Url: https://codereview.chromium.org/2831383002 .
Cr-Commit-Position: refs/branch-heads/3071@{#120}
Cr-Branched-From: a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed: https://chromium.googlesource.com/chromium/src/+/8041f5964cd3de050c62498f25ec559373412161

[awdf, 2017-04-21 16:55:44]

Description was changed from

==========
[Android O] Refactor channel definitions into new class

- As discussed, we can give this a per-file OWNERS for added safety
in case of channels being updated without increasing the version number.

- In future there will also be a requirement to move deprecated channels
into a new map, and thus a further need for safety.

- This refactoring will also make testing easier as fake channel
definitions can now be easily supplied in tests.

Review-Url: https://codereview.chromium.org/2832433002
Cr-Commit-Position: refs/heads/master@{#466316}
(cherry picked from commit 8aacc8f8d02e5b19f491b34f744085dd2c521167)

BUG=710843
==========

to

==========
[Android O] Refactor channel definitions into new class

- As discussed, we can give this a per-file OWNERS for added safety
in case of channels being updated without increasing the version number.

- In future there will also be a requirement to move deprecated channels
into a new map, and thus a further need for safety.

- This refactoring will also make testing easier as fake channel
definitions can now be easily supplied in tests.

Review-Url: https://codereview.chromium.org/2832433002
Cr-Commit-Position: refs/heads/master@{#466316}
(cherry picked from commit 8aacc8f8d02e5b19f491b34f744085dd2c521167)

BUG=710843

Review-Url: https://codereview.chromium.org/2831383002 .
Cr-Commit-Position: refs/branch-heads/3071@{#120}
Cr-Branched-From:
a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed:
https://chromium.googlesource.com/chromium/src/+/8041f5964cd3de050c62498f25ec...
==========

[awdf, 2017-04-21 16:55:45]

Committed patchset #1 (id:1) manually as
8041f5964cd3de050c62498f25ec559373412161.