[Thread] Add support for Thread persistent setting

The ThreadPersistentSetting class can be used to read/store key value
pairs in ThreadPersistentSetting.xml file.

Bug: 299243765

Test: atest ThreadNetworkUnitTests:com.android.server.thread.ThreadPersistentSettingTest
Change-Id: I564ce8373e6af8f5cdf066a579bec46bfffecb72