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

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

Bug: 122332514
Test: atest -p cts/tests/signature
Change-Id: Ib1c45dd788f2c9c3771eac958ac2d2c892cdbf86
1 file changed