[SettingsProvider] remove in-lock calls to PackageManager.getPackageUid()

Cache a list of package names of system packages, so we don't have to
check with package manager everytime if a package is system package or
not. This removes the deadlock between SettingsProvider and
PackageManagerService caused by getPackageUid().

BUG: 183007597
Test: run "atest NeneTest" and see there is no deadlock caused by
SettingsProvider

Change-Id: Ia98d47f12d218a7b98ef9003b786ba60e9212e20