libcutils: fs_config internal table alias test

Private interface to permit testing only added to fs_config to
expose android_files and android_dirs.

Make sure that both paths to a partition are specified in fs_config
internal tables.

Test: gTest libcutils-unit-test --gtest_filter=fs_config.*
Bug: 37703469
Change-Id: Ida5fccdb786dc6d67325005d4fdd1fa1ffaef396