Support /system_ext partition

This commit is mainly from I7a6a30bf8e8db9f2738594d187bb9148f138b8da, so
test cases and features are mostly same.

We change product_services partition name to "system_ext" because this
partition's purpose changes.

 - installing a RRO package for framework from /system_ext/overlay
 - installing apps from /system_ext/app
 - installing priv-apps from /system_ext/priv-app
 - installing permissions from
   /system_ext/etc/[default-permissions|permissions|sysconfig]

Bug: 134359158
Test: `mma` under frameworks/base/tests/[libs|privapp]-permissions
      adb sync && adb reboot
      adb shell cmd package list libraries
        => confirmed com.android.test.libs.system_ext library
      adb shell cmd package dump \
        com.android.framework.permission.privapp.tests.system_ext
        => confirmed that the package is a priv-app

Change-Id: Ibbccbba64156a7bc464ffb3785fb8fe69ebb973c