asoc: Set and get voice UI port indices on a per session basis

When multiple voice UI sessions are running, voice UI port
index might be overwritten with index of different sessions
during concurrent LPI and Non LPI session switches. Maintain
voice UI port index on a per session basis to avoid this issue.

Change-Id: I6b47c562e36795aff37a42fe10ae0c7c660bb82b
Signed-off-by: Aditya Bavanari <abavanar@codeaurora.org>