Fix bugprone-unused-return-value clang-tidy issues

This CL fixed the last remaining bugprone-unused-return-value
issue reported by clang-tidy: instead of calling the method
Runtime::AppendToBootClassPath (which does not get ownership
of the pointers to the DEX files), a new method called
Runtime::AddExtraBootDexFiles has been created, which works
similarly to the previous one, but calls internally the method
ClassLinker::AddExtraBootDexFiles, where pointers are
transferred ownership, therefore eliminating the for loop
with the calls to std::unique_ptr::release, which was most
likely causing a memory leak.

Bug: 213953102
Test: art/test/testrunner/run_build_test_target.py -j80 art-test | grep "936-search-onload"
Change-Id: I4b4237f3fb4fbe399e8cd3154e0465b9cfb0e05c
4 files changed