android: raise minSdkVersion to 21

The number of MPD installs on Android < 5.0 is negligible, and that
API version introduces lots of useful features for MPD.

src/input/plugins/FileInputPlugin.cxx

@@ -65,12 +65,9 @@ OpenFileInputStream(Path path,
 		throw FormatRuntimeError("Not a regular file: %s",
 					 path.c_str());
 
-#if !defined(__BIONIC__) || __ANDROID_API__ >= 21
-	/* posix_fadvise() requires Android API 21 */
 #ifdef POSIX_FADV_SEQUENTIAL
 	posix_fadvise(reader.GetFD().Get(), (off_t)0, info.GetSize(),
 		      POSIX_FADV_SEQUENTIAL);
-#endif
 #endif
 
 	return InputStreamPtr(new FileInputStream(path.ToUTF8().c_str(),