Bump Android tooling defaults to latest versions in MSBuild

[JonDouglas]

There are three Android SDK version MSBuild properties that we should bump in Xamarin.Android.Common.targets:

• $(AndroidSdkBuildToolsVersion)
• $(AndroidSdkPlatformToolsVersion)
• $(AndroidSdkToolsVersion)

Here are the current defaults:

<AndroidSdkBuildToolsVersion Condition="'$(AndroidSdkBuildToolsVersion)' == ''">23.0.0</AndroidSdkBuildToolsVersion>
<AndroidSdkPlatformToolsVersion Condition="'$(AndroidSdkPlatformToolsVersion)' == ''">25.0.5</AndroidSdkPlatformToolsVersion>
<AndroidSdkToolsVersion Condition="'$(AndroidSdkToolsVersion)' == ''">25.2.5</AndroidSdkToolsVersion>

Here is the latest versions as of Android 8.1:

• build-tools: 27.0.3
• platform-tools: 27.0.1
• sdk tools: 26.1.1

Currently there are some scenarios in which these defaults get picked up and used within various CI / CD services such as AppCenter. Instead of picking up old values, we should set these to be more recent.

[pjcollins]

A duplicate concern related to build tools version was discussed in https://bugzilla.xamarin.com/show_bug.cgi?id=60915. I will close that bugzilla issue as this item is now being tracked here.