floss: Move buffer for decoded PCM out of decoder

We need to be able to modify the decoded PCM frames in order to
reconverge samples after we conduct a packet loss concealment. This
change is to facilitate such follow-up implementation.

Also change to pass mSBC packet starting from sync word as the decoder
doesn't actually need the H2 header part.

Bug: 232463744
Tag: #floss
Test: Build and verify HFP WBS works
BYPASS_LONG_LINES_REASON: Bluetooth likes 120 char lines

Change-Id: I430860f1e6d066e65d16ca9e8d4ef0b24aa2df32