audio: Convert getDevicesForStream to return a DeviceTypeSet

As a pre-requisite for switching from 'int' to
'AudioDeviceDescription' in the framework AIDL interfaces,
get rid of the usage of 'int' as a mask of 'audio_devices_t'
values. This is needed because 'AudioDeviceDescription'
can't be coerced into an int directly, and interpreting
a mask of multiple 'audio_devices_t' values is ambiguous
due to presence of multi-bit device types.

Note that at the Java layer the returned DeviceTypeSet
is still collapsed into a single integer value. This needs
to be addressed separately.

Bug: 188932434
Test: check audio on device
Change-Id: I487d8144f10fce4d5353d50e46cc44e67ab6ff3b