Migrate GCM Registration Service from JobIntentService to WorkManager

[ravishanker]

Fixes #16015

This PR migrated JobIntentService to WorkManager for GCM token registration at the startup, and authn stages

JobIntentService is deprecated, and known to cause crashes on low end devices, on below Android 10.

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To Test:

Test 1

• install WP/JP app
• Login
• Verify, it works as before

Test 2

• Logout
• Login again
• Verify, works as before

Test 3

• Delete the app
• Fresh install the app
• Go through signup
• Ensure, everything works as before

Test 4

• If you have a low end device with Android 10 or below, try reproducing the crash in the issue with current version, and then test with this version to see if it fixes the issue

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Regression Notes

1. Potential unintended areas of impact

   • Login/Signup

2. What I did to test those areas of impact (or what existing automated tests I relied on)

   • Tested login, signup, and fresh install

3. What automated tests I added (or what prevented me from doing so)

   • Existing unit tests

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PR Submission Checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr19701-086d1b5
	Commit	086d1b5
	Direct Download	jetpack-prototype-build-pr19701-086d1b5.apk

Note: Google Login is not supported on these builds.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr19701-086d1b5
	Commit	086d1b5
	Direct Download	wordpress-prototype-build-pr19701-086d1b5.apk

Note: Google Login is not supported on these builds.

[irfano]

Thank you for taking care of this, Ravi! I believe this will significantly improve our crash-free metrics. 🚀
I tested it by attempting to receive push notifications in your test cases, and I successfully received them. Push notifications still work, and the diff LGTM! 👍🏻
I won't merge it now as I've added minor comments for you to review.

[wpmobilebot]

	3 Warnings
⚠️	Class GCMRegistrationScheduler is missing tests, but unit-tests-exemption label was set to ignore this.
⚠️	Class GCMRegistrationWorker is missing tests, but unit-tests-exemption label was set to ignore this.
⚠️	Class Factory is missing tests, but unit-tests-exemption label was set to ignore this.

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