Update min requirements from Android 5 to Android 7

[shepherd-l]

Description

One Line Summary

Update min requirements from Android 5 to Android 7

Details

Motivation

ClassNotFoundException: java.util.function.Consumer error in Android 5 and 6 from the Unity SDK's java bridge.

There doesn't look to be a straightforward way to support/workaround the java.util.function.Consumer class in Unity for Android 5 and 6. And since these Android versions are pretty old with around 2.3% global usage and will eventually be unsupported - we've decided to bump the minimum Android version of the Unity SDK to Android 7 where java.util.function.Consumer is supported.

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
  • If it is hard to explain how any codes changes are related to each other then it most likely needs to be more than one PR
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
  • Simplify with less code, followed by splitting up code into well named functions and variables, followed by adding comments to the code.
• I have reviewed this PR myself, ensuring it meets each checklist item
  • WIP (Work In Progress) is ok, but explain what is still in progress and what you would like feedback on. Start the PR title with "WIP" to indicate this.

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This change is 

[nan-li]

Do you have any stats (industry wide or OneSignal user base) to include in this PR about the prevalence of Android 5 and 6 to reinforce the version deprecation?

[shepherd-l]

Good question

Saw that Android 5 and Android 6 have about a 2.3% global usage, I'll clarify it in the PR description