Revert "Only allow the system server to connect to sync adapters"

This reverts commit 4df2de83.

There was a bug in the CL. Rather than fixing it forward, let's revert
it and fix it in a follow-up CL, so we'll only have to back-port
a single CL.

Bug: 203229608
Test: N/A, a follow-up CL will redo this change with a Test: line.

Change-Id: I5fddf283ace09996a2e5cfc272f8e9fd8168a114