Add an api_files property in java_api_library

java_api_contribution is useful to java_api_library when the api files
are not colocated.

If they are colocated (e.g. in Multi-tree assembled api_surfaces
directory), it is useful to refer to them directly without nedding to
create a java_api_contribution module.

Test: In build/soong, go test ./java
Change-Id: I5b4e557068a1e5c71a80c76452030e72ec83a696
Merged-In: I5b4e557068a1e5c71a80c76452030e72ec83a696

java/java_test.go

@@ -1691,6 +1691,7 @@ func TestJavaApiLibraryAndProviderLink(t *testing.T) {
 			name: "bar2",
 			api_surface: "system",
 			api_contributions: ["foo1", "foo2"],
+			api_files: ["api1/current.txt", "api2/current.txt"]
 		}
 		`,
 		map[string][]byte{
@@ -1708,7 +1709,7 @@ func TestJavaApiLibraryAndProviderLink(t *testing.T) {
 		},
 		{
 			moduleName:         "bar2",
-			sourceTextFileDirs: []string{"a/foo1.txt", "b/foo2.txt"},
+			sourceTextFileDirs: []string{"a/foo1.txt", "b/foo2.txt", "api1/current.txt", "api2/current.txt"},
 		},
 	}
 	for _, c := range testcases {