bootstat: Disambiguate boot time on encrypted devices.

This change introduces new metrics to discern boot time on encrypted devices:
* post_decrypt_time_elapsed
* boot_decryption_complete
* boot_complete_post_decrypt
* boot_complete_no_encryption

Bug: 27497357
Change-Id: I8d3a411029bd6a45f80589ff67c408593e133a87
(cherry picked from commit c08e996a)