Move AndroidOs to non-inner class

ForwardingOs isn't available in host-side tests because it's a special
class in LibCore that only exists on-device.

When ActivityThread.class is loaded, it will automatically load all
inner classes including this static one. The class loading will fail
because the superclass, ForwardingOs.class, won't exist.

Pushing the class load to runtime allows the tests to load the
ActivityThread.class by itself for mocking, without triggering a load
of the non-existent ForwardingOs.class.

Flag: EXEMPT mechanically moving code
Test: atest ActivityThreadTest
Bug: 362981229
Change-Id: I128822335d62814d959d4f19d363207954629c8a