GenericSource should report -1 instead of 0 for unknown duration

If the underlying source never reported a duration, GenericSource
would report a duration of 0. The right value to return is -1.

Bug: 21391417
Change-Id: Ie53045f819727b0ac79ed6719991724c069d7650
diff --git a/media/libmediaplayerservice/nuplayer/GenericSource.cpp b/media/libmediaplayerservice/nuplayer/GenericSource.cpp
index 88a7745..95f0709 100644
--- a/media/libmediaplayerservice/nuplayer/GenericSource.cpp
+++ b/media/libmediaplayerservice/nuplayer/GenericSource.cpp
@@ -56,7 +56,7 @@
       mVideoLastDequeueTimeUs(0),
       mFetchSubtitleDataGeneration(0),
       mFetchTimedTextDataGeneration(0),
-      mDurationUs(0ll),
+      mDurationUs(-1ll),
       mAudioIsVorbis(false),
       mIsWidevine(false),
       mIsSecure(false),