Rely on pthread to implement reader-writer lock for Android

Now that we support API Level 14 (Ice Cream Sandwich) and later only, we can safely assume that pthread is available.

This is just a performance optimization.  No user-visible change is intended.

BUG=none
TEST=none

git-svn-id: https://mozc.googlecode.com/svn/trunk@531 a6090854-d499-a067-5803-1114d4e51264