Audio V4: Cast conversion now deduce both types

That avoids to repeat the destination type which
can change between versions.

Bug: 38184704
Test: compile
Change-Id: I55c881afe5f6fc511ea01b19c5d46de12fb1e0d2
Merged-In: I55c881afe5f6fc511ea01b19c5d46de12fb1e0d2
Cherry-picked from master
Signed-off-by: Kevin Rocard <krocard@google.com>