Device-wide unknown sources block option for DPC.

This adds a new framework user restriction that can be used by the DPC
to block installs from unknown sources on all profiles of a device.

Test: Manual test, disallowing installs in TestDPC disables installing
unknown sources apps.
Bug: 111335021
Change-Id: Ib9fb672c5e5dea2ac63bf8cbd1b04484b12b4056
8 files changed