Return the right thing from Context.getApplicationContext()

Previously, this would always return the app context for the first application
ever instantiated in the process.  Usually this is fine, since usually there
will be one process per application, but in the case of shared-process apps,
it causes activities/views to try to access the wrong set of resources, be
unable to access their proper derived-class Application objects, etc.

Fixes bug #2513901

Change-Id: Ie19f2a86583c5d927db4d84a4262077450e37539