Export some api files for use by cts/tests/signature tests

Creates a filegroup for each of these files:
* api/current.txt
* api/system-current.txt
* api/system-removed.txt
* test-base/api/current.txt
* test-mock/api/current.txt
* test-runner/api/current.txt

Bug: 122332514
Test: atest -p cts/tests/signature
Change-Id: Ib031dcc39d3c1cca6dcdf977c0277d5e4875591e