Add build config symbol for multi audio focus

Define a new symbol for multi audio focus and default the value to
false. In MediaFocusControl, when
Settings.System.MULTI_AUDIO_FOCUS_ENABLED is not set, default to the
build default value.

Bug: 404907583
Bug: 416963897
Test: atest AudioFocusTest
Flag: android.media.audio.audio_focus_desktop
Change-Id: Ic5436ef6623042723531977e74606f89f609d686