Clear policy results and stats when setting or changing an installer

[lucasmrod]

Follow up PR for #21428.

After some discussions with Noah we want to clear policy results when a user sets for the first time or changes an installer on a policy.

[getvictor]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.36%. Comparing base (3541ad6) to head (79a8304).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #22053      +/-   ##
==========================================
+ Coverage   55.82%   65.36%   +9.53%     
==========================================
  Files        1494     1494              
  Lines      116479   116503      +24     
  Branches     3540     3540              
==========================================
+ Hits        65030    76157   +11127     
+ Misses      45202    33255   -11947     
- Partials     6247     7091     +844     

Flag	Coverage Δ
backend	66.69% <100.00%> (+10.59%)	⬆️

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