audio policy: fix preferred device for mmap I/Os

In findPreferredDevice(), when handling mmap I/Os, only clients for
the considered strategy/capture preset must be retained.

Bug: 345044066
Bug: 338143144
Flag: EXEMPT bug fix
Test: repro steps in the bug
Change-Id: Ie4efbf3a6d2c2651c81a1aabd82e3f42a62f8d39