Add APIs to allow Device Admins to change SELinux settings

These calls, added to the Device Admin API, will allow Device Admin apps
to change various SELinux settings, including:
* Toggling SELinux enforcing and permissive modes
* Toggle SELinux booleans
* Load a new SELinux policy file (sepolicy)
* Load new SELinux context files ({property,file,seapp}_contexts)

In order to use these APIs, a Device Admin must first request
USES_POLICY_ENFORCE_SELINUX, then become a SELinux Admin by calling
setSELinuxAdmin(). All other set* calls relevant to SELinux are guarded
by a check against whether the admin is a SELinux Admin.

Otherwise, the style of the set* calls are very similar to the other
calls setting device policy in the Device Admin API. That is, these
calls change the Admin's internal state and then call a sync method to
update the device's state to the Admin's state.

Change-Id: Ibc41464210a85493e9acf48763215f1714a58c34