Fix libdexfile dependencies to avoid direct dependency on libdexfile_external.

It should be dlopen'ed lazily by libdexfile_support now.

Also change debuggerd_test to not link libunwindstack and its dependencies
statically - the static libs can overlap with the dynamic ones.

Test: mmma system/core/debuggerd/
Test: atest debuggerd_test
Test: mmma system/core/{libunwindstack,libbacktrace}, run host gtests (cannot get atest to work)
Bug: 124827589
Bug: 123186083
Change-Id: I9e7bf9bcbae499af4e1be4c9854bce441e2a7b55