Update public documentation for C++ FCM SDK.

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

As of Firebase C++ SDK 7.1.0, we started using JobIntentService for scheduling jobs. This change requires a few additional modifications to AndroidManifest.xml and a new way of starting a background service.

These changes were done to the old repository, and need to be brought to GitHub.

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Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[jonsimantov]

LGTM, should this get a release note for the next release?

[a-maurice]

LGTM, should this get a release note for the next release?

I don't think it is necessary, none of this ends up in a build, it is just there for sample purposes.

[github-actions]

✅  Integration test succeeded!

Requested by @a-maurice on commit 483c74a
Last updated: Mon Dec 6 16:37 PST 2021
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