Add ability to observe arrival independently

[kmadsen]

Description

We recently finished controlling arrival #2787

But have noticed that we still want the ability to "observe" arrival, without needing to change the behavior. This pull request adds the ability to add/remove arrival observers.

• I have added any issue links
• I have added all related labels (bug, feature, new API(s), SEMVER, etc.)
• I have added the appropriate milestone and project boards

Testing

Please describe the manual tests that you ran to verify your changes

• I have tested locally (including SNAPSHOT upstream dependencies if needed) through testapp/demo app and run all activities to avoid regressions
• I have tested via a test drive, or a simulation/mock location app
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have updated the CHANGELOG including this PR

[codecov-io]

Codecov Report

Merging #2855 into master will increase coverage by 0.02%.
The diff coverage is 52.38%.

@@             Coverage Diff              @@
##             master    #2855      +/-   ##
============================================
+ Coverage     35.41%   35.43%   +0.02%     
- Complexity     2101     2103       +2     
============================================
  Files           544      545       +1     
  Lines         19541    19555      +14     
  Branches       1841     1842       +1     
============================================
+ Hits           6920     6929       +9     
- Misses        11794    11800       +6     
+ Partials        827      826       -1