Revert "gralloc: Allow unmapping concurrent handles"

This reverts commit a3a9aaa5bbf0a94f1039e3eda368fa8a28ddecf8.
gralloc: Attribute buffer ownership to last caller

Switch buffer management from creator to latest registrant

Change-Id: If6b1ab1a553f701e4a1038c089f2b2ecb02c2e8b