flexibility for having extra policy files

add an interface to accept a list of additional policy files, instead of just a single file.
Also add TEST_MAPPING for presubmit.

Bug: 147914640
Test: boot with empty, 1, and 2 element lists, atest
Change-Id: I97e8e0ec7b68699838595c09a4a8e7c7eef657c3
Merged-In: I97e8e0ec7b68699838595c09a4a8e7c7eef657c3
(cherry picked from commit 11c1a687)