Change the name of source files to replace "vpx" with "aom".
Change occurences of "vpx" in the code to "aom".
Change-Id: I002c0eaa2c350cf1a895bf01bea5788c61328027
Right now only IVF format is supported which is enough for example code.
Other formats like y4m, webm, raw yuv will be supported later.
Change-Id: I34c6f20731c1851947587ca5c589d7856b675164