Fix issue #6579824: Email crash observed after updating...

...device from JRN59D to JRN60

Deal correctly with multiprocess content providers that need to
be loaded into a secondary process; wasn't correctly detecting the
case where the IContentProvider returned by the activity manager
is null.  (installProvider used to be given the direct IContentProvider,
now it gets the ContentProviderHolder and much check whether the
provider inside is null.)

Change-Id: I888622e275a459031ab849952941f39cf9c02ee0