Framework: Prefer android.system.Os over libcore.io.Libcore.os

This is a pure refactoring with no a behavior change other than
that these calls now go through android.system.Os, which immediately
delegates to Libcore.os.

This is a first step towards separating framework (via
android.system.Os) vs. libcore (via Libcore.os) clients of these
low level APIs. Separating these is a prerequisite towards moving
the API parts of android.system into framework, and moving the
rest into a different package in libcore.

Test: Treehugger
Bug: 67901714

Change-Id: Ifd8349ec5416e5693f40dba48fdf2bef651b7d81
Merged-In: Ifd8349ec5416e5693f40dba48fdf2bef651b7d81