Implement support for ALT and SHIFT modifiers

The spec for keymaps and keboard input at
http://source.android.com/porting/keymaps_keyboard_input.html
mentions the stand alone ALT and SHIFT modifiers:

SHIFT: While pressed, the shift key modifier is set
ALT: While pressed, the alt key modifier is set

This commit implements support for these.

Change-Id: I5854ef3df541740cc3443b474a9c41183eb7561c
Ex: key 305   BACK  ALT