GL: Treat unknown AHardwareBuffer formats as RGBA8. 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/+/1816300 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Commit-Queue: Geoff Lang <geofflang@chromium.org>