[incfs] Notify IncrementalService about system startup later

It needs system to be ready to bind to services, which is much
later in the boot process

Bug: 151241369
Test: boot up Android, make sure no error messages from
  IncrementalService trying to bind a DataLoader when the OS
  can't bind services yet
Change-Id: Ibcef3f405965ee07c5b8adbb41f068be734bde9e