Match prev_ to number_of_layers
Match prev_ to number_of_layers

Defined as unsigned in VP8_CONFIG

Cleans warning in Android build:
comparison of integers of different signs: 'unsigned int' and 'int'
if (cpi->oxcf.number_of_layers != prev_number_of_layers)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~
Change-Id: I969e64cd2bfda6e61c564476dbd35b892b177646

luctrudeau/aom,mwgoldsmith/vpx,luctrudeau/aom,webmproject/libvpx,mwgoldsmith/libvpx,Topopiccione/libvpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,luctrudeau/aom,mwgoldsmith/vpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,mbebenita/aom,luctrudeau/aom,webmproject/libvpx,mwgoldsmith/libvpx,mbebenita/aom,mbebenita/aom,Topopiccione/libvpx,GrokImageCompression/aom,GrokImageCompression/aom,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,mbebenita/aom,smarter/aom,mwgoldsmith/vpx,ShiftMediaProject/libvpx,smarter/aom,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,smarter/aom,GrokImageCompression/aom,mbebenita/aom,Topopiccione/libvpx,smarter/aom,mwgoldsmith/vpx,mwgoldsmith/vpx,smarter/aom,mwgoldsmith/libvpx,GrokImageCompression/aom,mbebenita/aom,Topopiccione/libvpx,mwgoldsmith/vpx,luctrudeau/aom,webmproject/libvpx,mbebenita/aom,GrokImageCompression/aom,mbebenita/aom,Topopiccione/libvpx,ShiftMediaProject/libvpx,smarter/aom,luctrudeau/aom,Topopiccione/libvpx,mbebenita/aom,GrokImageCompression/aom