audio policy: fix input preferred device logic

In AudioPolicyManager::getNewInputDevice(), the intention is to allow
and app to specify a preferred device only if no other app is capturing.
The implemented rule was actually preventing an app from specifying a
preferred device if that same app was also capturing.

Bug: 379233370
Test: repro steps in bug
Flag: EXEMPT bug fix.
Change-Id: I6d4b8a461e49088fa2bfb1ec25af8c96caa5d979