Bug 1112020 - The build script of keyboard app would fail when copying
the a specific ime folder for the 3rd time.
 - Stop copying the same ime folder when it already exists.
Merge pull request #26817 from RudyLu/keyboard/Bug1112020-copy_ime_folders

Bug 1112020 - The build script of keyboard app would fail when copying the a
r=timdream.