Added supports-split-screen-multi-window command to am

The supports-split-screen-multi-window command allows the ability to
check if the device has split screen multiwindow support in shell. The
other changes allows the code to determine if the device has support for
it as well.

This will also help CTS tests to determine if the device has split
screen before trying to run the test.

Test: manual - adb shell am supports-split-screen-multi-window
Change-Id: I3da32b5b03712a8537104469d67100ec796de9e6