Donot compose layers if external is in pause state

* Donot compose external/virtual layer-list when
when external/virtual is in pause state.
* Marking the layers for GPU composition when external/virtual
is in pause state would update FB_TARGET layer which inturn would
affect partial mdpcomp after resume.

Change-Id: Id2806bd7156085c00158a31dff3c3dc4c2b61146
hwc: Make the fallback to Copybit composition a compile time option

- let only the devices that actually need it to use this code path
  (it breaks the transition animations on S4 Plus devices otherwise)

Change-Id: Ie6dadf5f0b32631c355eb41e6a0e77ed4e08f763