[Spec] Add IsBackButtonEnabled to Popup API

[SkyeHoefling]

Add IsBackButtonEnabled to Popup API

In the Popup API there is no way to specifically control Android's hardware back button and how it interacts with the popup. Currently if the hardware back button is used the popup will act as if it is light dismissed. We should add granular control to turn the hardware back button on and off.

Add new attached property for android that enables/disables the hardware back button for dismissing Popups

API

Popup - Android Attached Property

Properties

API	Description
IsBackButtonEnabled	Gets or sets if the hardware back button is enabled. If true the hardware back button will light dismiss the popup. If false the hardware back button won't cause any action to the popup.

This should be implemented as an attached property

Events

N/A

Scenarios

Platform Compatibility

• Target Frameworks:
  • iOS: N/A
  • Android: ✅
  • UWP: N/A

Backward Compatibility

N/A

Difficulty : low

[SkyeHoefling]

I investigated the code for this and hardware back is disabled on android if LightDismiss is disabled. I didn't think this was working correctly when I wrote the issue. I also had some difficulty disabling the hardware back but leaving light dismiss. I think it would be uncommon to have the granularity between hardware back and light dismiss.

In my investigation I see we use the following APIs

SetCanceledOnTouchOutside(false);
SetCancelable(false);

I attempted to use these independently of each other but if the 2nd one is set to false it appears to override any value used for the 1st one.

I do think we need to update the XML docs for the Light Dismiss property as this is not explained and it can be confusing. I'll submit a PR to clean up the XML docs which I'll mark fixing this issue