ImageWriter: Do not override UNKNOWN format twice

Currently when application calls ImageWriter.newInstance(Surface, int,
int), the default format of UNKNOWN is overridden to surface format in
Java side before being overridden again to user specified format (from
RGBA to IMPLEMENTATION_DEFINED). This breaks backward compatibility with
certain gralloc implementations.

Do not override UNKNOWN format in Java side.

Test: testWriterFormatOverride
Bug: 143637345
Change-Id: Ie61157bca11f623eff2b8c0ed2a988c86da07cb2