Build updatable-bcp-packages.txt from prebuilts in sdk snapshot

Previously, the java_import and java_sdk_library_import modules did not
make their permitted_packages available to the rule that generates the
updatable-bcp-packages.txt file. This change corrects that.

Bug: 193763688
Test: m nothing
      - Added unit tests, which all failed and then fixed the tests.
Change-Id: If0706e4551a331b48d383123088e63924dded48b

apex/platform_bootclasspath_test.go

@@ -398,6 +398,7 @@ func TestPlatformBootclasspath_AlwaysUsePrebuiltSdks(t *testing.T) {
 			name: "foo",
 			prefer: false,
 			shared_library: false,
+			permitted_packages: ["foo"],
 			public: {
 				jars: ["sdk_library/public/foo-stubs.jar"],
 				stub_srcs: ["sdk_library/public/foo_stub_sources"],