audio: Add recommended mute duration attribute to APM XSD

Add 'recommendedMuteDurationMs' optional attribute to 'mixPort'
element. This attribute specifies customized mute duration while
switching device's audio path to avoid truncated sound at the
beginning.

The framework is already aware of this attribute, only need
to allow it in the XSD.

Bug: 183344508
Bug: 197823000
Test: m
CTS-Coverage-bug: 215647214
Change-Id: I3be36009d1b5db218c846efe96f078e858283992