[Wi-Fi DPP] Refine code

Remove unused code.
Rename some variables and methods for better readability.
Fix typos.
Reduce unnecessary variable scope and method scope.
Simplify some code logic.

Bug: 120645817
Test: atest com.android.settings.wifi.dpp
Change-Id: I8f30b3e445f25ada927e995befee1783d2270b91