Fix blur sampling outside bounds of layer

The image that the blur shader uses is the screenshot of the whole
screen, translated to the start of the blurRect. However, nothing stops
the shader from sampling outside of the right/bottom edges of the
blurRect. As a result, the blurring takes samples from outside of the
layer bounds.

This CL restricts the blur shader to sample the blurRect edge color when
it tries to sample outside the blurRect.

Bug: 167166562
Test: atest SurfaceFlinger_test -- --test-arg
com.android.tradefed.testtype.GTest:native-test-flag:"--gtest_filter=Layer*Tests/Layer*Test.SetBackgroundBlurRadiusSimple*"

Change-Id: I5f8f7c4a87cbcd3b0cb080720e554119d8d07f69