Add comments to Asus Gamepad button mappings

Currently, the circle button on odie (Asus Gamepad) is mapped to HOME.
But it is not clear which button is which just from looking at the key
layout file.
Add some annotations here.

Bug: 111431828
Bug: 110270125
Test: atest AsusGamepadTestCase (the test is currently in development)
Change-Id: I8d1317be7f403ceaf0c2d72d756623e3cd032559