Add java_boot_libs to sdk

The build has some implicit dependencies (via the boot jars
configuration) on a number of modules, e.g. core-oj, apache-xml, that
are part of the java boot class path and which are provided by mainline
modules (e.g. art, conscrypt, runtime-i18n) but which are not otherwise
used outside those mainline modules.

As they are not needed outside the mainline modules adding them to
the sdk/module-exports as either java_libs, or java_header_libs would
end up exporting more information than was strictly necessary. This
change adds the java_boot_libs property to allow those modules to be
exported as part of the sdk/module_exports without exposing any
unnecessary information.

Some points to note:
* The java_import has to have a valid file for the src property
  otherwise it will be disabled.
* The src property is supposed to reference a jar file but the
  java_boot_libs property will make it reference an empty file (not
  an empty jar) so that any attempt to use that file as a jar, e.g.
  compiling against it, will cause a build failure.
* The name of the file passed to the src property should make it
  clear that the file is not intended to be used.
* The test makes sure that only the jar file is copied to the
  snapshot.

Test: m nothing
Bug: 171061220
Change-Id: I175331e4c8e3874ab70a67cdc2f76ed1576e41eb

sdk/java_sdk_test.go

@@ -472,6 +472,61 @@ aidl/foo/bar/Test.aidl -> aidl/aidl/foo/bar/Test.aidl
 	)
 }
 
+func TestSnapshotWithJavaBootLibrary(t *testing.T) {
+	result := testSdkWithJava(t, `
+		module_exports {
+			name: "myexports",
+			java_boot_libs: ["myjavalib"],
+		}
+
+		java_library {
+			name: "myjavalib",
+			srcs: ["Test.java"],
+			java_resources: ["resource.txt"],
+			// The aidl files should not be copied to the snapshot because a java_boot_libs member is not
+			// intended to be used for compiling Java, only for accessing the dex implementation jar.
+			aidl: {
+				export_include_dirs: ["aidl"],
+			},
+			system_modules: "none",
+			sdk_version: "none",
+			compile_dex: true,
+		}
+	`)
+
+	result.CheckSnapshot("myexports", "",
+		checkAndroidBpContents(`
+// This is auto-generated. DO NOT EDIT.
+
+java_import {
+    name: "myexports_myjavalib@current",
+    sdk_member_name: "myjavalib",
+    visibility: ["//visibility:public"],
+    apex_available: ["//apex_available:platform"],
+    jars: ["java_boot_libs/snapshot/jars/are/invalid/myjavalib.jar"],
+}
+
+java_import {
+    name: "myjavalib",
+    prefer: false,
+    visibility: ["//visibility:public"],
+    apex_available: ["//apex_available:platform"],
+    jars: ["java_boot_libs/snapshot/jars/are/invalid/myjavalib.jar"],
+}
+
+module_exports_snapshot {
+    name: "myexports@current",
+    visibility: ["//visibility:public"],
+    java_boot_libs: ["myexports_myjavalib@current"],
+}
+
+`),
+		checkAllCopyRules(`
+.intermediates/myexports/common_os/empty -> java_boot_libs/snapshot/jars/are/invalid/myjavalib.jar
+`),
+	)
+}
+
 func TestHostSnapshotWithJavaImplLibrary(t *testing.T) {
 	result := testSdkWithJava(t, `
 		module_exports {