This fixes the following mismatch bugs:
* At the bitstream level, the decoder would not read the delta_qindex
  information for supertx blocks, but the encoder always sent it,
  leading to the encoder and decoder becoming misaligned.
  The delta_qindex information is still required for supertx blocks,
  so change the decoder to read it.
* In addition, the quantizer was not properly adjusted for supertx
  blocks at the decoder. We copy the quantizer setup code from
  non-supertx blocks.

Since this does not change the encoder, it should not have any
quality impact.

Change-Id: I9a0f79c3aa66f2a5a353821e2a6f3b526636e7b4