PixelCopyTest: More modification to the edge detection area.

Previous change from the same motivation (fp16 precision gpu)
  https://android-review.googlesource.com/c/platform/cts/+/952398/
does not look working on different screen resolutions/densities.

This CL changes the sampling point of 'edge' to 1 pixel outer
to avoid picking different colors due to low precision.

Bug: 148645349
Test: android.view.cts.PixelCopyTest pass
Change-Id: Ib67920ac89ddcc30e321c772b973c069dd0f2ee5