Since radioext has already moved to this folder, it doesn't make sense
for the base folder to be dump_modemlog. This change moves it to its
own subfolder so that we can also add more in the future.
Test: build and flash, trigger bugreport and check modem logs are there
Bug: 302435001
Change-Id: Ia83378074068526023f591d63b1e5ac4700b8103
