[res] Modernize the code in StringPool

Use the C++20+ features like concepts and std::erase

+ fix a bug in Prune() where it wasn't removing the strings that
  were only referenced in the span names of the just-removed
  style

Test: build + atest liandroidfw_tests aapt2_tests
Flag: EXEMPT refactoring
Change-Id: I7275449f98c8e00142217e5ee3d86b1d84e51069