Returns AccountHash from get_filler_account()

[brooksprumo]

Problem

AccountsDb::get_filler_account() returns the account's hash as a bare Hash. We have AccountHash now, and should use that instead.

Summary of Changes

Return AccountHash from get_filler_account().

[codecov]

Codecov Report

Merging #33758 (83d3494) into master (93d882f) will decrease coverage by 0.1%.
Report is 2 commits behind head on master.
The diff coverage is 89.7%.

@@            Coverage Diff            @@
##           master   #33758     +/-   ##
=========================================
- Coverage    81.8%    81.8%   -0.1%     
=========================================
  Files         806      806             
  Lines      217908   217911      +3     
=========================================
- Hits       178270   178267      -3     
- Misses      39638    39644      +6     

[yhchiang-sol]

LG!