Add ExternalTexture class into RenderEngine interface

ExternalTexture is an RAII structure that wraps raw GraphicBuffers that
are passed into RenderEngine. ExternalTexture's role is to help with
managing GPU resources of GraphicBuffers by mapping buffers into
textures, EGLImages, or AutoBackendTextures depending on the
RenderEngine backend. Under the hood, mapExternalTextureBuffer and
unmapExternalTextureBuffer (renamed from cacheExternalTextureBuffer and
unbindExternalTextureBuffer respectively) are used to help tie
resource management to the ExternalTexture lifetime.

The main motivation for this is that currently managing buffer
lifecycle has historically been errorprone and caused memory leaks, so
this improves code health.

As part of this:
* mapExternalTextureBuffer and unmapExternalTextureBuffer
are now protected methods, and are never called outside of RenderEngine
with the exception of creating and destroying ExternalTextures.
* Because GLESRenderEngine's output buffers are cached differently from
Skia RenderEngine, if there are output-only buffers then disable the
mapExternalTextureBuffer calls whenever GLESRenderEngine is used.
* Custom RAII classes in the Planner and in BufferLayerConsumer are now
removed since they're subsumed by ExternalTexture
* RenderSurface now controls its own management of ExternalTextures in a
small queue
* cleanFramebufferCache is now unimplemented for Skia, because
ExternalTextures are now deleted whenever a RenderSurface is deleted.

Bug: 180767535
Test: libsurfaceflinger_unittest
Test: libcompositionengine_test
Test: librenderengine_test
Test: Simulate virtual displays
Test: Screen reotation
Test: Movie playback on Google TV
Test: Force GPU composition
Test: screenshot

Change-Id: I222c71e6e1c67485cdeac49e2cb829289af9efec