Add $(AndroidSdkPlatformToolsVersion) MSBuild property, update $(AndroidSdkBuildToolsVersion)

[jonpryor]

Add $(AndroidSdkPlatformToolsVersion) MSBuild property, update $(AndroidSdkBuildToolsVersion) to be consistent.

[jonpryor]

@dellis1972: There was also talk on the call about adding an NDK version as well, probably $(AndroidNdkVersion). Alongside this fix would be a good time to add it. :-)