Bug 4016329 do full string comparisons

Use full string comparisons instead of partial for
  file extension and MIME type.
Do case-insensitive comparison of MIME type and file extensions.
Fix error in comment for String8::getPathExtension.
Remove dead code -- StringTokenizer is unused.

Change-Id: I322be6235abbdaab5f7eafa48926dbb2cf46dc29
diff --git a/include/utils/String8.h b/include/utils/String8.h
index 6b49ff5..4163697 100644
--- a/include/utils/String8.h
+++ b/include/utils/String8.h
@@ -165,8 +165,8 @@
     String8 walkPath(String8* outRemains = NULL) const;
 
     /*
-     * Return the filename extension.  This is the last '.' and up to
-     * four characters that follow it.  The '.' is included in case we
+     * Return the filename extension.  This is the last '.' and any number
+     * of characters that follow it.  The '.' is included in case we
      * decide to expand our definition of what constitutes an extension.
      *
      * "/tmp/foo/bar.c" --> ".c"