keep balance when override account

[yihuang]

Closes: #342

Description

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #353 (20ecd70) into main (30cc004) will increase coverage by 15.60%.
The diff coverage is n/a.

❗ Current head 20ecd70 differs from pull request most recent head a610e8f. Consider uploading reports for the commit a610e8f to get more accurate results

@@             Coverage Diff             @@
##             main     #353       +/-   ##
===========================================
+ Coverage   52.42%   68.03%   +15.60%     
===========================================
  Files          46        9       -37     
  Lines        4614     1198     -3416     
===========================================
- Hits         2419      815     -1604     
+ Misses       2093      358     -1735     
+ Partials      102       25       -77     

Impacted Files	Coverage Δ
x/evm/keeper/params.go
x/evm/keeper/abci.go
types/account.go
x/evm/types/codec.go
x/evm/types/query.pb.gw.go
x/evm/types/chain_config.go
types/config.go
x/evm/types/storage.go
x/evm/types/logs.go
types/codec.go
... and 26 more

[yihuang]

https://github.com/yihuang/web3-integration-tests/blob/master/tests/test_statedb.py#L21

verified in this integration test.

[leejw51crypto]

lgtm