audio: Fix incorrect userId for setMicMute

Within the manager class, we should be using the context userId, not
a binder callingUserId.

While this works if we are not in a binder context, it could break if
we get a binder call from another user id.

Test: Compiles
Bug: trivial
Flag: EXEMPT trivial
Change-Id: I442111b2acd01b55096baa7daa170a3df8199a24