Make 4 PackageManagerService APIs lock-free

Bug: 178754214

The following public PackageManagerService APIs are handled by the
snapshot computer.
 * filterAppAccess
 * getSigningDetails

Compared the times from startUser, stopUser, and switchUser to the
baseline and saw no regression.

Test: atest
 * FrameworksServicesTests:UserSystemPackageInstallerTest
 * FrameworksServicesTests:PackageManagerSettingsTests
 * FrameworksServicesTests:PackageManagerServiceTest
 * FrameworksServicesTests:AppsFilterTest
 * FrameworksServicesTests:PackageInstallerSessionTest
 * FrameworksServicesTests:ScanTests
 * UserLifecycleTests#startUser
 * UserLifecycleTests#stopUser
 * UserLifecycleTests#switchUser

Change-Id: Ic5c707f2d65f427883d578b822fe9968e584f42c