configs: add system property to set AudioFlinger client heap size

Playback of high sample rate multichannel clips fails when
repeat mode is set to repeat one. This is because the default
heap size allocated for AudioFlinger clients is 4MB which is insufficient.

Set the system property ro.af.client_heap_size_kbyte to 7KB. This property
is read in AudioFlinger and multiplied by 1024, so effectively the allocated
heap size becomes 7MB which is sufficient to play such clips in repeat mode.

CRs-fixed: 2128229
Change-Id: Ie5e970d44331216a4cbf24498277459c0be71958