Android: Use '/' in INSTALL_ROOT if sh.exe is in PATH

Task-number: QTBUG-31147

Change-Id: Ifdeef2fa96668fdb482fd613eb8766c1ff5e3afd
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>