Add 'audio/mp3' to android.mime.types

MimeTypeMap.getSingleton().getExtensionFromMimeType() returns null since
MimeTypeMap doesn't have 'audio/mp3'. It makes
FileUtils#buildUniqueFile() returns a wrong file. For example, when
mimetype is 'audio/mp3' and filename is test.mp3 which already exists,
it returns test.mp3 (1).

Bug: 205774136
Test: atest FrameworksCoreTests:android.os.FileUtilsTest
Test: atest CtsMimeMapTestCases
Change-Id: I201a7955c735fe99813b5a99692525ace7b0f5fe