refactor: activity service & bug fixes

[muktihari]

• Fix bug when handling multiple sessions FIT File that missing session's Lap causing program to panic. Test using FIT files listed here: https://github.com/muktihari/fit/tree/master/testdata/from_garmin_forums
• Simplify logic and ensure correctness
• Add package mem's unit test
• Bump FIT SDK Version from v0.16.4 to v0.19.0, overview changes:
  • Fix various bugs on previous versions
  • SDK Profile is updated to the latest version v21.141 (Release Date: May 15, 2024)
  • Performance improved for both decoding and encoding, especially encoding is now a lot faster.
• Fix bug when sorting the resulting messages on Activity's ToFIT method, the messages was not properly sorted when encountering a message without timestamp or a message with invalid timestamp. Now, we use the FIT SDK for Go's built-in sorting function for a more robust sorting algorithm.

[muktihari]

I've added some changes to this PR, please kindly review when you have time @raditzlawliet. Thank you!