Draw rectangle shadow fast in low elevation cases

When the elevation is not high, the shadow is similar
to just add some dark lines on the edges and corners of
rectangle views. Using simple algorithm can also improve
the performance and save more memory.

Test: The render result of unit test is updated.
Bug: 37906145

Change-Id: I28758f868ee6e24e4552368bddfb7ac10fe0a205
2 files changed