Make snapshotctl logging better.

snapshotctl merge --logcat --log-to-file
- If --logcat, log to logcat
- If --log-to-file, log to /data/misc/snapshotctl_log/
- If both, log to both
- If none, log to stdout

Test: manually test these 4 cases
Bug: 148818798
Change-Id: I44b52936c0d095867acc6ee781c6bec04f6ebd6b