Exit boot animation cleanly.

The desc.txt file can now mark parts as 'must finish cleanly' by using
'c' as the part line prefix rather than 'p'.  If so indicated, if the
bootanimation is asked to quit it will do so only after waiting to
finish that part.

I considered either making init.c service killing smarter or promoting
bootanim to be a bindable service with a requestExit method.  However,
these changes are probably too big/risky given our ship date.  So
I used a property as a mailbox between SurfaceFlinger and bootanim.

Bug: 6679877
Change-Id: I1f8dd9e7da1ea80a483b31fa14c4a5645922d774