audio: Make renamings suggested by reviewers

Rename DriverInterface::getPosition -> refinePosition. This reflects
the fact that the driver may provide more precise position, however
this is an optional operation.

Rename StreamIn/Out::mContext -> mContextInstance. This is
to emphasize that StreamIn/Out actually own the context and they
hold the real context instance, not just a reference/pointer
to it, like other classes involved in the inheritance chain.

Bug: 286914845
Test: atest VtsHalAudioCoreTargetTest
Change-Id: Id7159f9bf3ee020dc7cf594ec35f5d59d23d7e31