Support native platform Http handlers for Xamarin.iOS and Xamarin.Android

[peombwa]

This PR adds support for native http handler when targeting Xamarin.iOS (NSUrlSessionHandler) and Xamarin.Android (AndroidClientHandler).

Fixes #

• #481 Use native HTTP client handlers for Xamarin.iOS and Xamarin.Android.
• #482 Skip CompressionHandler when NSUrlSessionHandler is used as final handler.

Changes proposed in this pull request

• Updates core SDK to target Xamarin.iOS10 and MonoAndroid70.
• Adds preprocessor directives for Xamarin.iOS and Xamarin.Android that are used to wire up native http handlers for the respective platform.
• Adds an internal RemoveHandler method to GraphClientFactory to simplify removal of a DelegatingHandler by type.
• Updates unit tests.
• Adds Native Xamarin.Android test runner project to run existing unit tests on Android.

Other links

• Perf issues with HttpClientHandler when targeting Xamarin.iOS and Xamarin.Android Cannot download multiple OneDrive files concurrently since v.1.13.0-beta msgraph-sdk-dotnet#413.
• NSUrlSessionHandler limitations NSUrlSessionHandler - using GZipStream causes IOException anaisbetts/ModernHttpClient#50 (comment).
• .Net SDK TFMs limitations Support additional target frameworks such as for Xamarin, UWP, etc dotnet/sdk#491.
• We can't add a Xamarin.iOS test runner project due code generation restriction imposed on the platform, thus Moq based tests can't run.
• .Net Standard implementation support https://docs.microsoft.com/en-us/dotnet/standard/net-standard#net-implementation-support.

[MIchaelMainer]

Add release notes

[darrelmiller]

typo

Suggested change

	LongRunnungOperationHandler = 0x00000100
	LongRunningOperationHandler = 0x00000100