Isolate SpriteIcon into its own file.

This is to isolate differences between internal master and AOSP so that
the open source intern can work on a relatively new project.

It also isolates a divergent piece of code into SpriteIcon.cpp.

Bug: None
Test: Pointer icons in ApiDemos still work.
Change-Id: I1bf8f23411ad7170de2497d6fee0f70b015e9b5f
Merged-In: I1bf8f23411ad7170de2497d6fee0f70b015e9b5f