Modifying dpm.setSecureSetting call for install_non_market_apps

Starting from O, install_non_market_apps is deprecated and will not be
checked by the package installer. Device admin apps should be using the
user restriction instead.
Since on managed profiles, the default value blocked install from
unknown sources, the system will set the user restriction on behalf of
the profile owners (if the profile has one).
For non-managed profiles, the user had access to the settings to change
the value of install_non_market_apps. So going forward, any request to
change it's value by dpm#setSecureSetting in such users is going to be
ignored.

Test: Manually tested that:
1. For a profile with PO, when install_non_market_apps was set to 0,
user restriction is set on upgrade
2. For a profile with PO, when install_non_market_apps was set to 1,
user restriction is not set on upgrade
3. After upgrade, newly created managed profiles with PO have user
restriction set

Bug: 33947615
Change-Id: I063e9ee608b52086ffdf8ed2b24e2928574c58cd