…er Impeller/GLES. (#44734)

- Refactor AndroidExternalTextureGL into SurfaceTextureExternalTexture
base class with two specialized implementations:
- Add SurfaceTextureExternalTextureGL for legacy GL rendering.
- Add SurfaceTextureExternalTextureImpellerGL for Impeller GLES
rendering.

NOTE: There is still no support for SurfaceTextures under Vulkan
rendering.

Adds the SLSA L1 compliance badge for the engine.

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2023-08-15 [email protected] [bazel] //gm/BazelGMRunner.cpp: Simplify how Gold keys are generated.
2023-08-15 [email protected] Remove deprecated Metal GrBackendRenderTarget constructor
2023-08-15 [email protected] Move SDFTextRenderStep logic into modules.
2023-08-15 [email protected] Move BitmapTextRenderStep logic into modules.
2023-08-15 [email protected] //gm/gm.h: Fold GM::onShortName() into GM::getName() and make it const.

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Adds missing tests for `CompositionAwareMixin`. Check #44139 (comment) and #44139 (comment)

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