partially fix [3306150] HTML5 video with H/W acceleration blackout (DO NOT MERGE)

We used to guarantee that a layer in SurfaceFlinger would never be
destroyed before all references (to its ISurface) on the client
side would be released. At some point, this guarantee got
relaxed to allow to free gralloc resources sooner. This last
change was incorrect, because:
- in implementations with reference-counting the gralloc resources
wouldn't be released anyways, until all the mapping were gone
- in implementations without ref counting, the client side
would most likely crash or do something bad
- it also caused the SharedBufferStack slot to be reallocated
to another surface, which could be problematic if the client
continued to use the surface after the window manager destroyed it.

So, we essentially reinstate the guarantee that layers won't be
destroyed until after all references to their ISurface are
released.

NOTE: This doesn't entirely fix 3306150 because there is another
problem there where the Browser continues to use a surface after it
has been destroyed.

also improve SurfaceFlinger 'dumpsys' log

list the purgatory, which shows windows that have been closed,
but for which the client still has references.

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