[Android] local variable 'xpk_temp_dir' referenced before assignment.

In the exception catch code block, 'xpk_temp_dir' is used before assignment.
It would mislead the develper with wrong log. So assign 'xpk_temp_dir'
with empty string at the beginning to fix this issue.

BUG=https://github.com/crosswalk-project/crosswalk/issues/999
Merge pull request #1001 from junmin-zhu/xpk_dir

[Android] local variable 'xpk_temp_dir' referenced before assignment.