Expose blur, Draw a ring outside of the given size in RippleShader.

Design doc can be found in the bug attached.

This is part of polishing the tablet docking animation. Note that
the below CLs will be submitted together under the same topic:
* [this CL] Expose blur, draw a ring outside (to match the logic with
  the other shapes)
* Add TargetSize (name subject to change)
* Update parameter values in WirelessChargingLayout

Bug: 269124200
Test: RippleViewText, Manual
Change-Id: Idf5c7838491149b8f240c91413db2dc7ce166479