A2DP: Add mandatory elements for SBC SRC capabilities

* The SBC items below are mandatory for SBC SRC capabilities
(see A2DP_SPEC_V13.pdf, Section 4.3.2):
 - Channel Mode - mono
 - Block Length - 4, 8, 12, 16
* Updated/fixed the corresponding unit tests

Bug: 63260735
Test: Manual - A2DP SBC streaming
Change-Id: I04c4f0b830411bc736d7c402b86006c182af02e7