GH-309: Fixes #309, update to set activity on init and oncreate, never null.

[jamesmontemagno]

Description of Change

Fixes up handling activity changes. Helpful in fragments and sets activity right away.

Bugs Fixed

• Related to issue No Exception on Android if Activity is null (due to init not being set) #309 & Java.Lang.NullPointerException: Attempt to invoke virtual method 'void android.app.Activity.requestPermissions #289

Behavioral Changes

None

PR Checklist

• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of master at time of PR
• Changes adhere to coding standard
• Updated documentation (see walkthrough)

[dend]

✅ Validation status: passed

File	Status	Preview URL	Details
Xamarin.Essentials/Platform/Platform.android.cs	✅Succeeded

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