Some code refactoring.

Move unique id generation logic into separate file.
Move createSettingsPage function to Spp.
Add getPageProvider API in Spp for ease use.
Add NullPageProvider instance.

Bug: 244122804
Test: unit-test & local build gallery
Change-Id: Ic99ab6a99db98d0666a4c5c59d8841e18d628eeb