Speed up slider switch layout by using native extractAlpha() method

Was making use of this great widget, but having performance problems on layout.  I found that most of the time was being spent in onLayout() iterating over the background mask to extract the alpha layer to a separate mask drawable.  Replacing that loop with a call to Bitmap.extractAlpha() eliminated the performance issues I was having.

Sample layout times measured on my Nexus One:
Before: 1.6 Seconds
After : 7 milliseconds