Make ConfigWriter simpler

- Remove varg WriteLine
- No prefix
- Add WriteVars for writing multiple lines using "="/ "+="

This refactoring is a prep for removing "ResolveVariables()" which is very slow.

Bug: 149020353
Test: atest --test-mapping system/linkerconfig
Change-Id: I6bf2204a9ac0794b0bdf8028999a3543821a26be
7 files changed