Do not clean up global/system settings on package unintalls.

Legacy apps can write their own entries in the system settings and
when they get uninstalled these are hanging around forever polluting
the settings table. We keep track of which settings an app added and
when the app is uninstalled we drop its custom entries. The trouble
was that we did the same thing for global and secure settings with
no explicit list of platform defined settings. Hence, if say a test
signed by the platform certificate touches platform defined global
or secure settings and is then uninstalled, we would drop the platform
defined entries portentially crippling the system.

bug:20113160

Change-Id: Ia21694f6326ad4a1795c4666027b366e26c05a23