When Android's AImageReader encounters YUV formats, it converts the
hardware buffer's internal format to vendor-specific formats.  Since
these sample as RGB using samplerExternal, simply validate them as if
they are RGB8.

BUG=angleproject:3929

Change-Id: I991512c8a584b08a978aacf67ab153ada7e4fd76
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1816300Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>