slice() the ByteBuffer passed to ImageDecoder

Bug: 73788928
Test: I7d5082ba7319c6c069dde5d0efb22af6e92dd243

This way the input's position is unaffected. Update the docs to reflect
the new behavior.

Change-Id: I7212948ee289ea8da1be9fe81d3f4bc9296e3e61
1 file changed