`conv_linker_config proto -s` should work with multiple json input
files, but ParseDict() overwrites list fields (e.g. provideLibs), not
appending elements.
Also added a test.
Bug: 264330513
Test: conv_linker_config_test
Change-Id: Idc482f941201f15e5fc276c0ffc0dfeaa09d0cc2