On Android, MotionEvent with 0 values might come for axes when connecting
a gamepad, even though a user didn't touch any axes. This is problematic
especially for tirgger axes, where the default value should be -1.
This change fixes the issue by adding a new state `axesReady` to check
if an axis is really touched or not. If an axis is not touched yet,
a button value for a standard (trigger) button always returns 0.
This change also removes an old hack to initialize axis values for
triggers.
Closes#2598
Before this fix, the button and axis IDs are from the OS. These
didn't match with the SDL game controller databaes unfortunately.
This fix changes the assignments of the buttons and the axes to match
with the database.
Closes#2312
