Update libsuperuser
Don't crash when phone is not rooted

Closing the shell throws a NPE when it couldn't be opened in the first
place. We could detect the by checking for SHELL_EXEC_FAILED explicitely,
but there might be other exceptions which shouldn't cause crashes.
Handle libsuperuser's SHELL_DIED result
Add background for installation mode text

This should give at least a little hint that it is clickable. Surely not
best solution, but better than nothing. This will become obsolete with a
redesign soon anyway.

The image is the old AOSP toast background with reduced padding.
Bump version to 2.5 beta2