cmTarget: Track internally whether platform is Android

Add an IsAndroid member to save whether CMAKE_SYSTEM_NAME is "Android".
Add 'ANDROID_GUI' target property to mark Android applications

Also add a 'CMAKE_ANDROID_GUI' variable to set the property default
so a project can easily make all executables Android applications.
An Android application executable file has the same extension as a
shared library (.so).
Add 'ANDROID_API' target property to set Android Target API

Also add a 'CMAKE_ANDROID_API' variable to set the property default.
VS: Generate ANDROID_GUI executables as app packages

When an executable is marked with ANDROID_GUI, generate an Ant build.xml
file for it and enable the AntBuild step in the .vcxproj file.