stack/a2dp: De-duplicate identical helper functions

IsSourceCodecValidxxx, IsSinkCodecValidxxx, IsPeerSourceCodecValidxx,
IsPeerSinkCodecValidxxx all have the same implementation; there
is no reason for keeping all of these helpers.

Bug: 331817295
Test: m com.android.btservices
Flag: EXEMPT, no logical change
Change-Id: I5d20952be54443b18be772bb70b9e56c5f9ad8ba