o/devicestate, o/snapstate: move firstboot configure task after essential snap installs

[ernestl]

This PR builds on #13117 and primarily on #13097.

Implements draft spec parts:

• Trivial seed install task order
• Seed install task order
• Preseed install task order

Outstanding: testing, spread and unit tests

[codecov-commenter]

Codecov Report

Merging #13127 (463ace0) into master (7c17cf2) will increase coverage by 0.03%.
Report is 50 commits behind head on master.
The diff coverage is 82.88%.

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@@            Coverage Diff             @@
##           master   #13127      +/-   ##
==========================================
+ Coverage   78.80%   78.83%   +0.03%     
==========================================
  Files        1011     1013       +2     
  Lines      125739   126150     +411     
==========================================
+ Hits        99085    99451     +366     
- Misses      20442    20480      +38     
- Partials     6212     6219       +7     

Flag	Coverage Δ
unittests	78.83% <82.88%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
overlord/snapstate/snapstate.go	84.20% <80.48%> (-0.18%)	⬇️
overlord/devicestate/firstboot.go	85.57% <81.67%> (-3.61%)	⬇️
overlord/snapstate/flags.go	100.00% <100.00%> (ø)
overlord/state/task.go	95.94% <100.00%> (+0.17%)	⬆️

... and 22 files with indirect coverage changes

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