Avoid blurring and rounded corners without EGL_EXT_protected_content

If the GPU does not support protected content, avoid forcing client
composition for protected layers. This means removing rounded corners.
It also means avoiding blurring when it would require blurring a layer
with protected content.

Bug: 196271643
Test: manual (Netflix, YouTube, ExoDefault) with modified
      RenderEngine::supportsProtectedContent()
Change-Id: I9763eb22884e611568b36b2e221ee6d75ec3363e