Make sync adapters singletons.

We saw multiple sync adapter objects concurrently, which is BAD.
If service.onCreate got called while a sync was in flight, this
could happen (and all evidence pointed to this being the case),
so let's just not let that happen at all.

Bug: 10150344
Change-Id: Ia914bd256aa05ccb86cfa5203e18df5a03aa34be
4 files changed