Prevent clickjacking of superuser requests

Enable filterTouchesWhenObscured on the accept request button to prevent
possible clickjacking attacks. This is a measure used in many sensitive
dialogs (app installation, backup & VPN) in AOSP.

Change-Id: I33c18c449c909b97b29b981bffe7f3095adee506
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/Superuser into cm-11.0