Flag to control if generated SDK prebuilts are preferred.

This is needed because:
- the SDK prebuilt drop is being automated, including the Android.bp
- We want to control the prefer flag on a per-module level

It augments the existing SOONG_SDK_SNAPSHOT_PREFER which now overrides
this new flag when set to true.

Bug: 188427719
Test: m nothing
Change-Id: Ieb5ab9fab53a34c615345b7a9d19cadf713eec26

sdk/sdk_test.go

@@ -662,3 +662,68 @@ sdk_snapshot {
 		)
 	})
 }
+
+// Ensure that sdk prebuilt_prefer works correctly.
+func TestSnapshot_PrebuiltPreferTrue(t *testing.T) {
+	bp := `
+		sdk {
+			name: "mysdk",
+			java_header_libs: ["myjavalib"],
+			prebuilts_prefer: true,
+		}
+
+		java_library {
+			name: "myjavalib",
+			srcs: ["Test.java"],
+			system_modules: "none",
+			sdk_version: "none",
+			compile_dex: true,
+			host_supported: true,
+		}
+	`
+	preparer := android.GroupFixturePreparers(
+		prepareForSdkTestWithJava,
+		android.FixtureWithRootAndroidBp(bp),
+	)
+
+	checkZipFile := func(t *testing.T, result *android.TestResult, expected string) {
+		zipRule := result.ModuleForTests("mysdk", "common_os").Rule("SnapshotZipFiles")
+		android.AssertStringEquals(t, "snapshot zip file", expected, zipRule.Output.String())
+	}
+
+	t.Run("prefer=true", func(t *testing.T) {
+		result := android.GroupFixturePreparers(
+			preparer,
+		).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: true,
+    visibility: ["//visibility:public"],
+    apex_available: ["//apex_available:platform"],
+    jars: ["java/myjavalib.jar"],
+}
+
+sdk_snapshot {
+    name: "mysdk@current",
+    visibility: ["//visibility:public"],
+    java_header_libs: ["mysdk_myjavalib@current"],
+}
+			`),
+		)
+	})
+}