frameworks/base: Add dock keyboard support for Asus Transformer tablets. 

Based on patches by jruesga from CyanogenMod sources. This commits adds
support for the Asus Transformer dock special keys. For this commit to 
work, kernel and device tree commits are needed to implement the device 
specific handling

Removed duplicate device specific key handler function code.

Change-Id: Ie6d3a22c549ddc15056c5e99d16b194f799f75d1