hwc: vds: Add support to dump layers, FB and WB output buffer

1. Add hook to dump the layers going to VDS based virtual display.

2. Furthermore, add utility function to dump the frame buffer and
   WB output buffer. This will be enabled via a system property

   debug.hwc.enable_vds_dump

   Once dumping is enabled, the dumps can be extracted from

   /data/vds.fb.FORMAT.XRESxYRES.raw
   /data/vds.wb.FORMAT.XRESxYRES.raw

   e.g. vds.fb.MDP_RGBA_8888.1152x1920.raw

Change-Id: I2435c5507961a52929411206892c005492d5c193