Set Secure User ID from app level.

When AndroidKeyStore keys require used authentication, they need to be
bound to a Keymaster's Secure User ID. This ID will be set by keystore
soon. Until then, set it from the framework level (i.e., from apps
which use AndroidKeyStore).

NOTE: Accessing gatekeeper to obtain the Secure User ID will be
blocked by SELinux policy. To test this code, disable SELinux
enforcing mode.

Bug: 18088752
Change-Id: I7a3315eb52f0fc978d14d5d0e9613f2f36c6c01e