PackageManager lock reduction: server reconfiguration

Bug: 161323622

Modify selected PackageManagerService functions to prepare then for
the snapshot feature.  In each function, code that is locked by the PM
lock is extracted into its own function.

Test: atest
 * FrameworksServicesTests:UserSystemPackageInstallerTest
 * FrameworksServicesTests:PackageManagerSettingsTests
 * FrameworksServicesTests:PackageManagerServiceTest
 * FrameworksServicesTests:AppsFilterTest
 * FrameworksServicesTests:PackageInstallerSessionTest
 * FrameworksServicesTests:ScanTests

Change-Id: Idb673c94d08a4b259c3f34bdb2f27703974553a1