Cleanup/document Shaders for UI rendering module

Bug: 156636925
Test: make

Remove UnsupportedAppUsage false-positives from Shader and BitmapShader.

Change public/package-protected methods that should only be called by
subclasses to protected and mark them @hide.

Document why other @hide methods must be @hide.

Remove unnecessary if statement in BitmapShader constructor.

Change-Id: If3e87c45cbbab1df180387dd6ccfda1ec2ecb153