Move starting filesystem checkpoint out of StagingManager.

And into apexd. Since we don't need filesystem checkpointing for APKs
that are part of a staged install, it makes more sense to do all of
the checkpointing logic in apexd instead.

Bug: 126740531
Test: atest apexservice_test
      atest apex_e2e_tests

Change-Id: If832bcfacd976ec173af1b0ffa73f4fdc6632cc4