Bug 1219475 - Fix a comparison warning. r=gerald

Coercing the shift to a bool makes the comparison useless.
This just affects whether a warning is printed, so there's
no change to what files we accept.

We also need to mask of the high 'Starts with SAP' bit
which is usually set and would replace our false negative
with a false positive. ISO/IEC 14496-12:2012(E) sec. 8.16.3.2

media/libstagefright/frameworks/av/media/libstagefright/MPEG4Extractor.cpp

@@ -2190,7 +2190,7 @@ status_t MPEG4Extractor::parseSegmentIndex(off64_t offset, size_t size) {
             ALOGW("sub-sidx boxes not supported yet");
         }
         bool sap = d3 & 0x80000000;
-        bool saptype = d3 >> 28;
+        uint32_t saptype = (d3 >> 28) & 0x3;
         if (!sap || saptype > 2) {
             ALOGW("not a stream access point, or unsupported type");
         }