Fix google-explicit-constructor warnings in system/media.

Declare explicit conversion constructors.

Bug: 28341362
Change-Id: I04ef1036731cd0d9d2d96b9dd9f111020b6b9ef5
Test: build with clang-tidy
diff --git a/audio_utils/spdif/BitFieldParser.h b/audio_utils/spdif/BitFieldParser.h
index 1cf45d0..3f6fe59 100644
--- a/audio_utils/spdif/BitFieldParser.h
+++ b/audio_utils/spdif/BitFieldParser.h
@@ -27,7 +27,7 @@
 class BitFieldParser {
 public:
 
-    BitFieldParser(uint8_t *data);
+    explicit BitFieldParser(uint8_t *data);
     virtual ~BitFieldParser();
 
     /**