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

test-mock/Android.bp

@@ -30,3 +30,15 @@ java_sdk_library {
     srcs_lib_whitelist_pkgs: ["android"],
     compile_dex: true,
 }
+
+// Make the current.txt available for use by the cts/tests/signature tests.
+// ========================================================================
+filegroup {
+    name: "android-test-mock-current.txt",
+    visibility: [
+        "//cts/tests/signature/api",
+    ],
+    srcs: [
+        "api/current.txt",
+    ],
+}