Don't import GPU inaccessible buffers in RenderEngine

SurfaceFlinger already rejects those buffers from being added to
drawLayers, but mapExternalTextureBuffer() is still called for all
buffers arriving into SurfaceFlinger. Let's not trigger undefined
behavior.

Bug: 300155347
Test: builds, boots
Change-Id: I3ced5f11f47af6fdcd2c1575cbbbac99075588b8