Change checkpointing retry count to 2

Changing the checkpointing retry count to 2 resolves
issues of apex sessions being reverted if a hard reboot takes
place during a userspace reboot. Previously, a revert would
occur based on what stage of the boot sequence a hard reboot
took place in, now the revert will not occur at all.

Bug: 148270009
Test: atest ApexRollbackTests
Change-Id: Idce2d60f0a25f01a7d081420c937e5c9cec12e5d