Add AudioChannelMask to Framework SAIDL definition

Framework SAIDL used to use 'int' for the channel mask
type. To help finding all the places where channel masks
are used, add a designated type. Currently it's an enum,
but it will be changed to a more extendable type soon (see
the bug and the plan document).

Bug: 188932434
Test: m
Change-Id: If2d958fcc92b8e549493d3fa83b35c12dcda2ccc