This reverts commit 2cb66715.

Reason for revert: Make DisplayGbm fail to compile. See: https://ci.chromium.org/ui/p/chromium/builders/try/linux-chromeos-compile-dbg/636833/overview

Original change's description:
> EGL: Merge DisplayAndroid into DisplayEGL.
>
> These two classes have a lot of duplicated code for no reason.
>
> Bug: angleproject:5563
> Change-Id: Ia9d8efeb50f3e52ffb1df6a6e462efdb20fd4a9a
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2636678
> Commit-Queue: Geoff Lang <geofflang@chromium.org>
> Reviewed-by: Peng Huang <penghuang@chromium.org>
> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>

Bug: angleproject:5563
Change-Id: Ic39071130727b37cc2a168d4b8e3c50f3e7184a5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2704051Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>