Simplify range check for audio_mode_t

AudioSystem::setMode previously allowed negative modes, but these were
then rejected by AudioFlinger.

Now negative modes (including AUDIO_MODE_INVALID and AUDIO_MODE_CURRENT)
are explicitly disallowed.

Change-Id: I0bac8fea737c8eb1f5b6afbb893e48739f88d745
diff --git a/media/libmedia/AudioSystem.cpp b/media/libmedia/AudioSystem.cpp
index f7f129c..84f6b7c 100644
--- a/media/libmedia/AudioSystem.cpp
+++ b/media/libmedia/AudioSystem.cpp
@@ -158,7 +158,7 @@
 
 status_t AudioSystem::setMode(int mode)
 {
-    if (mode >= AUDIO_MODE_CNT) return BAD_VALUE;
+    if (uint32_t(mode) >= AUDIO_MODE_CNT) return BAD_VALUE;
     const sp<IAudioFlinger>& af = AudioSystem::get_audio_flinger();
     if (af == 0) return PERMISSION_DENIED;
     return af->setMode(mode);