Use junit-hostdex instead of core-junit-hostdex/junit4-target-hostdex

A recent change in external/junit changes junit-hostdex so it
includes all JUnit classes not just the ones not in
core-junit-hostdex and not in junit4-target-hostdex. This change
updates this project to use junit-hostdex instead of either of
or even both of the other two.

Bug: 30188076
Test: make checkbuild and art/tools/run-libcore-tests.sh '--mode=host' '--variant=X32'
Change-Id: Ieaf9edb8cdd2306242386170688a8b5843b55470