1/2: Rename domain "system" to "system_server".

This is a follow-up CL to the extraction of "system_app" domain
from the "system" domain which left the "system" domain encompassing
just the system_server.

Since this change cannot be made atomically across different
repositories, it temporarily adds a typealias "server" pointing to
"system_server". Once all other repositories have been switched to
"system_server", this alias will be removed.

Change-Id: I90a6850603dcf60049963462c5572d36de62bc00