Uses a custom state description for the Audio Balance seek bar.

This makes the seek bar more informative for a user of a screen reader
or other accessibility service. See context in b/319575109#comment7.

Bug: 319575109
Flag: com.android.settings.accessibility.audio_balance_state_description
Test: Use TalkBack to observe the state description while moving the
      seek bar.
Test: atest BalanceSeekBarTest
Change-Id: I7be15b26116f83854efe69d6b0560edde946daf5