Fix flaky tests due to shared system property

The system property is shared within JVM, change system property in a
test case might break test cases in another test class. To address the
issue, introduce SystemProperty helper class to back up and restore the
system properties in tests.

Bug: 373177618
Flag: EXEMPT Test only
Test: atest SettingsRoboTests
Change-Id: I15539ce5ac425f35571d796baa25f259df1b601f