system/timezone: Prefer Os over Libcore.os

Since this package has access to framework, it can use
android.system.Os which is the logically "framework level"
abstraction, even though it currently lives in libcore.
This makes system/timezone more consistent with other
android.* / com.android.* code (which uses Os), separating
it from libcore code (which uses Libcore.os).

Test: Treehugger
Bug: 67901714

Change-Id: I5a852c22ad55a32157e896328fd14197f24878c5