Unbind texture buffer after each drawLayers call

This is to make sure that the source buffer gets removed from
the RenderEngine's image cache, otherwise it may leak memory.

bug: 162202251
test: CTS MetadataRetrieverTest; also test with local test app
that extracts many frames from hdr video clips, meminfo should
not show growing graphics memory before & after the app run.

Change-Id: Id970281732322da68a670a69bbe2f20a20881ebc