Increased max number of mixes per audio policy

This is needed for use cases where audio policy is used to setup
different set of audio configurations per user. Particularly useful for
automotive use cases, where the audio policy is used to defined 4
different audio zones, each having 10 different audio mixes. Each mix
defines where audio is routed for different audio attributes and users
combinations.

Bug: 262438891
Test: atest audiopolicy_tests
Change-Id: I092ca9144be3170c0fb7c7d284f3ba6bbcfe58e1