Add unit tests for libdexfile external API.

This prompted a fix to ExtDexFileMakeString in the C API to make it fully
support strings with NUL chars. That's currently not expected to be
required; it's for the sake of API completeness.

Test: m test-art-{host,target}-gtest-dex_file_supp_test
Test: art/test/testrunner/ -j80 art-test
Bug: 119632407
Change-Id: Ibd3ef6e3ea7ae196f541e926a4a7665be1c91819
6 files changed