Remove dependency on libdexfile internal header.

Also remove some tests for implementation details in the libdexfile API that
are now covered by unit tests there (http://r.android.com/867370).

Test: mmma system/core/{libunwindstack,libbacktrace} and run host gtests
Bug: 119632407
Change-Id: Ie731bb929a564de56ce1c78385da12a43d0954ff