Support generating sdk snapshots that can be selected by Soong config vars

Allows the builder of an sdk snapshot to specify a Soong config
variable that can be used to control whether the snapshot's prebuilts
are used or not.

Bug: 193523070
Test: m nothing
Change-Id: Ib09500ba61befc1202dff61dc06847c730ba9253

sdk/sdk_test.go

@@ -556,6 +556,49 @@ java_import {
     jars: ["java/myjavalib.jar"],
 }
 
+sdk_snapshot {
+    name: "mysdk@current",
+    visibility: ["//visibility:public"],
+    java_header_libs: ["mysdk_myjavalib@current"],
+}
+			`),
+		)
+	})
+
+	t.Run("SOONG_SDK_SNAPSHOT_USE_SOURCE_CONFIG_VAR=module:build_from_source", func(t *testing.T) {
+		result := android.GroupFixturePreparers(
+			preparer,
+			android.FixtureMergeEnv(map[string]string{
+				"SOONG_SDK_SNAPSHOT_USE_SOURCE_CONFIG_VAR": "module:build_from_source",
+			}),
+		).RunTest(t)
+
+		checkZipFile(t, result, "out/soong/.intermediates/mysdk/common_os/mysdk-current.zip")
+
+		CheckSnapshot(t, result, "mysdk", "",
+			checkAndroidBpContents(`
+// This is auto-generated. DO NOT EDIT.
+
+java_import {
+    name: "mysdk_myjavalib@current",
+    sdk_member_name: "myjavalib",
+    visibility: ["//visibility:public"],
+    apex_available: ["//apex_available:platform"],
+    jars: ["java/myjavalib.jar"],
+}
+
+java_import {
+    name: "myjavalib",
+    prefer: false,
+    use_source_config_var: {
+        config_namespace: "module",
+        var_name: "build_from_source",
+    },
+    visibility: ["//visibility:public"],
+    apex_available: ["//apex_available:platform"],
+    jars: ["java/myjavalib.jar"],
+}
+
 sdk_snapshot {
     name: "mysdk@current",
     visibility: ["//visibility:public"],