Dynamically load the libicu from the system. Android NDK doesn't ship with icu headers so for now define prototypes for all the functions we use ourselves.

We still need to hook up the native System.Security.Native library.

Unblocks Xamarin.Forms startup.

# Conflicts:
#	eng/native/configuretools.cmake
#	src/libraries/Native/Unix/CMakeLists.txt
#	src/libraries/Native/Unix/System.Globalization.Native/CMakeLists.txt
#	src/libraries/Native/Unix/System.Globalization.Native/configure.cmake
#	src/libraries/Native/Unix/System.Globalization.Native/pal_icushim.c

# Conflicts:
#	src/libraries/Native/Unix/CMakeLists.txt