Frameworks/base: Refactor UserHandle and Environment a bit

Refactor slightly to forbid using myUserId in the zygote (uid == 0).
Also factor Environment to put user data (vs shared data) into its
own class, which should keep the page the shared data is on actually
shared between processes.

Bug: 29338430
Change-Id: I05d1306b57658a83299e38076171f56cb364ea80