ART: Write bit fields together in ComputeSpecialAccessorInfo().

Avoid function calls between storing individual bit fields
to allow the compiler (gcc/clang) to merge those writes
together. Valgrind then marks the memory as "defined" while
individual bit field writes would leave it "undefined" and
later trigger the valgrind error:
  Conditional jump or move depends on uninitialised value(s)
on DCHECK()s using the bit fields.

Bug: 27552451
Change-Id: If6de5cbe231f99da0f974a0fc9a36c14e3dc071e