Swap contexts when unmapping ExternalTextures

Protected surfaces aren't visible to other contexts even when the
contexts have a shared object space, so the protected context must be
active when freeing protected textures. Otherwise, this may result in a
GPU memory leak.

Bug: 187870007
Bug: 189899352
Test: WidevineHEVCPlaybackTests
Change-Id: Id4d51df1878903a8e638c38c0cfb64527402143b