Added support for focal points in RadialGradient

Updated RadialGradient API to accept optional starting
(focal) point parameters. Internally create a two point
conical shader if focal points are provided.

Fixes: 167695230
Test: Added CTS test to verify focal points on RadialGradient
Change-Id: I18baba3d0a7285c3f787af8f441c797414bae2cf
4 files changed