Add API to create new contexts with custom configurations.

This allows you to, say, make a Context whose configuration
is set to a different density than the actual density of the device.

The main API is Context.createConfigurationContext().  There is
also a new API on ContextThemeWrapper that allows you to apply
an override context before its resources are retrieved, which
addresses some feature requests from developers to be able to
customize the context their app is running in.

Change-Id: I88364986660088521e24b567e2fda22fb7042819