RE: move useProtectedContext calls to drawLayers

This change removes a race condition when calling RenderEngine from
multiple threads. Before this change, two competing threads could call
RenderEngine::useProtectedContext with different values, potentially
causing RenderEngine::drawLayers to use the wrong context.

We currently call RenderEngine from a single thread but that will change
with the upcoming refactor of SurfaceFlinger::renderScreenImpl.

Bug: 238643986
Test: presubmits
Change-Id: I78ca00797b1604cedae1f134e963b6b78f2b802f