b/15721931
This follows an example pattern from GoogleHttpClient.
It tries to get the SSLCertificateSocketFactory from
GmsCore using reflection. If that fails, (because GmsCore
is not installed on the device) then it will fall back
to the platform implementation.
MailApplication sets a static object in SSLUtils that
allows it to get an externally created SSLCertifcateSocketFactory.
If this method is set, then it will use it, otherwise it
will fall back to the platform factory. This way there
is no reference to GmsCore in the AOSP email.
Change-Id: I0890fe4c3d79283fb98a4dc5a62a32efd320e52a
601700a61e