UserSystemPackageInstaller: Implicit System Mode

Previously, to ease local development, the SYSTEM user was always
implicitly whitelisted (meaning that, if the system package whitelist
didn't have an entry for a particular system package, it would *always*
be implicitly whitelisted on user 0). Here, we turn this into a
optional mode (implicitWhitelistSystemMode) set in config.xml.

Test: atest UserSystemPackageInstallerTest
Bug: 134605778
Fixes: 145611545
Change-Id: I5cf8eaa31969bafc4c3bbe15ecd7bd36f3a87384