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Problem:
AndroidSyncSettings (ASS) will soon depend on ProfileSyncService (PSS),
particularly in its constructor. Although this is is fine in production
code, SyncTestRule poses two issues for this setup on test code.
- ASS is created before a fake PSS provided by tests can be injected.
This is a problem because the initial state of PSS will be relevant to
the constructor logic of ASS, so we need to be able to fake this initial
state to write good tests.
- ASS is created before the JNI loads in startMainActivityForSyncTest().
For tests that don't provide a fake PSS, ASS will then attempt to talk
to the real PSS before JNI is loaded, causing an error.

Solution:
When SyncTestRule creates an ASS, the goal is actually just to fake one
of its internal dependencies, SyncContentResolverDelegate. In this
CL, we convert SyncContentResolverDelegate into a singleton. In this way,
a fake for this object can be injected without needing to early construct
ASS*. Furthermore, JNI is loaded earlier so that the rule can safely use
tools like FakeServer and FakeProfileSyncService.

* Note that constructing after startMainActivityForSyncTest() is not an
option because there's already code talking to ASS when this function
is running.

Bug: 1125622
Change-Id: I9dd938fd5d8211d58a97adb6d484c1a21a3b67db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424150
Commit-Queue: Victor Vianna <victorvianna@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810116}