stack::btm::sco_hci - interface with non-Floss build

This is a preparation for non-Floss build to utilize the mSBC
encoder/decoder. The function interfaces with audio stack require
different implementation than Floss ones.
Android specific implementations for these functions will be added
later and guarded by flag.

The existing Floss-only test cases in sco_hci_test.cc are now moved
to test target sco_hci_linux_test in a BUILD.gn used by Floss build.

Test: m com.google.android.btservices, verify HFP audio works in Floss
Test: atest --host net_test_stack_btm
Test: ./build.py in floss build
Bug: 349290628
Flag: EXEMPT, Floss-only changes
Change-Id: I00158a4e580379d6ea91d7661c36662cd49a29ce