Support /product-services partition

This CL is largely an adaptation of Change-Id
I16175933cebd9ec665d190cc5d564b5414a91827 . I also used the same way for
testing the change.

This CL will support the followings.
- installing a RRO package for framework from /product-services/overlay
- installing apps from /product-services/app
- installing priv-apps from /product-services/priv-app
- installing permissions from
  /product-services/etc/[default-permissions|permissions|sysconfig]

Bug: 80741439
Test: `mm` under frameworks/base/tests/[libs|privapp]-permissions
  adb sync && adb reboot
  adb shell cmd package list libraries
    => confirmed com.android.test.libs.product_services library
  adb shell cmd package dump \
    com.android.framework.permission.privapp.tests.product_services
    => confirmed that the package is a priv-app

  And I moved vendor/overlay/framework-res__auto_generated_rro.apk
  into system/product-services/overlay/ on taimen, and I confirmed that the
  RRO was installed properly.

Change-Id: I7a6a30bf8e8db9f2738594d187bb9148f138b8da
(cherry picked from commit a4af41736894bd3bf5bdc2a279acbeed2a24dd3d)
diff --git a/core/java/android/os/Environment.java b/core/java/android/os/Environment.java
index e32ed9d..347f60f 100644
--- a/core/java/android/os/Environment.java
+++ b/core/java/android/os/Environment.java
@@ -45,6 +45,7 @@
     private static final String ENV_ODM_ROOT = "ODM_ROOT";
     private static final String ENV_VENDOR_ROOT = "VENDOR_ROOT";
     private static final String ENV_PRODUCT_ROOT = "PRODUCT_ROOT";
+    private static final String ENV_PRODUCT_SERVICES_ROOT = "PRODUCT_SERVICES_ROOT";
 
     /** {@hide} */
     public static final String DIR_ANDROID = "Android";
@@ -67,6 +68,8 @@
     private static final File DIR_ODM_ROOT = getDirectory(ENV_ODM_ROOT, "/odm");
     private static final File DIR_VENDOR_ROOT = getDirectory(ENV_VENDOR_ROOT, "/vendor");
     private static final File DIR_PRODUCT_ROOT = getDirectory(ENV_PRODUCT_ROOT, "/product");
+    private static final File DIR_PRODUCT_SERVICES_ROOT = getDirectory(ENV_PRODUCT_SERVICES_ROOT,
+                                                           "/product_services");
 
     private static UserEnvironment sCurrentUser;
     private static boolean sUserRequired;
@@ -196,6 +199,16 @@
     }
 
     /**
+     * Return root directory of the "product_services" partition holding middleware
+     * services if any. If present, the partition is mounted read-only.
+     *
+     * @hide
+     */
+    public static File getProductServicesDirectory() {
+        return DIR_PRODUCT_SERVICES_ROOT;
+    }
+
+    /**
      * Return the system directory for a user. This is for use by system
      * services to store files relating to the user. This directory will be
      * automatically deleted when the user is removed.