refactor: simplify muxed account handling in Address and MuxedAccount classes

[overcat]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 96.87500% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.14%. Comparing base (9607f34) to head (0eacc69).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/main/java/org/stellar/sdk/StrKey.java	92.85%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #735      +/-   ##
============================================
+ Coverage     81.11%   81.14%   +0.03%     
- Complexity     1322     1324       +2     
============================================
  Files           214      214              
  Lines          5078     5082       +4     
  Branches        435      436       +1     
============================================
+ Hits           4119     4124       +5     
+ Misses          703      701       -2     
- Partials        256      257       +1     

Files with missing lines	Coverage Δ
src/main/java/org/stellar/sdk/Address.java	83.00% <100.00%> (+1.81%)	⬆️
src/main/java/org/stellar/sdk/MuxedAccount.java	83.33% <100.00%> (-3.34%)	⬇️
src/main/java/org/stellar/sdk/StrKey.java	91.69% <92.85%> (+0.06%)	⬆️

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