Settings change for device-wide unknown sources block.

Update the "Install unknown apps" preferences to display the option is
disabled if the global user restriction for unknown apps is turned on.

Test: Manual test, disallowing installs in TestDPC disables installing
unknown sources apps. Also:
atest packages/apps/Settings/tests/robotests/src/com/android/settings/applications/appinfo/ExternalSourcesDetailsTest.java
Bug: 111335021
Change-Id: I235e5fd110e296ed43aa89d7a5ec0d48470c4023