Update ClassLoaderContext to support artd use cases.

- Change `FlattenDexPaths` to returning `std::vector<std::string>`
  so that artd can use it in a more flexible way.

Bug: 229268202
Test: Presubmit
Change-Id: Iab0d018d4f52eea6b60f7f2d88cc56f23db6bd5c
Merged-In: Iab0d018d4f52eea6b60f7f2d88cc56f23db6bd5c
(cherry picked from commit 55e733f0f582f3edafa2b3bfd1168ade01813a54)
3 files changed