stagefright: fix finding hardware codec

When findMatchingCodecs(.., kHardwareCodecsOnly,..) was called,
it wouldn't actually match any hardware codecs because of a typo:

  if (!(TRUE_prefer_HW_codec && !FALSE_is_SW_codec))
    ==> if (!(TRUE && TRUE))

Fix logic and clarify statements

Change-Id: Ice725f043475caedd8f7ab961e0bc985db2eba6f