Make max fast tracks configurable using a property

ro.audio.max_fast_tracks

Rename the currently configured maximum number of fast tracks
from FastMixerState::kMaxFastTracks to FastMixerState::sMaxFastTracks.

There is no guarantee that the CPU will be able to handle
the configured number of fast tracks.

Bug: 27564141
Change-Id: If9af226d839b226503488c3cb20a4bb8950b429d
diff --git a/services/audioflinger/Tracks.cpp b/services/audioflinger/Tracks.cpp
index 61b30c1..9d430aa 100644
--- a/services/audioflinger/Tracks.cpp
+++ b/services/audioflinger/Tracks.cpp
@@ -402,7 +402,7 @@
         //mAudioTrackServerProxy->framesReadyIsCalledByMultipleThreads();
         ALOG_ASSERT(thread->mFastTrackAvailMask != 0);
         int i = __builtin_ctz(thread->mFastTrackAvailMask);
-        ALOG_ASSERT(0 < i && i < (int)FastMixerState::kMaxFastTracks);
+        ALOG_ASSERT(0 < i && i < (int)FastMixerState::sMaxFastTracks);
         // FIXME This is too eager.  We allocate a fast track index before the
         //       fast track becomes active.  Since fast tracks are a scarce resource,
         //       this means we are potentially denying other more important fast tracks from