Entry ClearButtonVisibility Handler

[bkaankose]

Description of Change

This PR implements ClearButtonVisibility property for iOS and Android.

Implements #379

Additions made

• Adds ClearButtonVisibility property to IEntry.
• Adds MapClearButtonVisibility method to EntryHandler.
• Adds IOnTouchListener and IOnFocusChangeListener to handle clear button's visibility in Android handler when the focus is changed and button is touched.
• Adds override for custom clear button drawable in Android handler.

Notes

• I needed to implement touch and focus listener to handle button's visibility in Android handler. This implementation might need to be reworked later on when a views or controls have general touch and focus listening mechanism (if they the team has plans to do so).

• Device tests are missing (WIP) but functionality work as expected for both iOS and Android.

• I don't know if this overriding approach in the handler for custom clear button drawable is correct. Open for suggestions.

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)
• Targets a single property for a single control (or intertwined few properties)
• Adds the property to the appropriate interface
• Avoids any changes not essential to the handler property
• Adds the mapping to the PropertyMapper in the handler
• Adds the mapping method to the Android, iOS, and Standard aspects of the handler
• Implements the actual property updates (usually in extension methods in the Platform section of Core)
• Tags ported renderer methods with [PortHandler]
• Adds an example of the property to the sample project (MainPage)
• Adds the property to the stub class
• Implements basic property tests in DeviceTests

[mattleibow]

Looks really clean and nice. Besides the tests you said you are still working on, there was one question more than a change.

[mattleibow]

@PureWeen Just pointing this out. Not sure what the official way to override things are like this. Should it be some mapper or some other registration that can be injected without requiring an override in a new type? We obviously need to be careful with having a bajillion mapper functions for everything. Maybe we need to clarify where the line of mapper/override/parameter is.

[bkaankose]

Device tests are implemented for both platforms.