Load splits on-demand

A split may be declared in an application's base manifest, but,
defined in a feature split. When resolving such a component,
invoke the installer to download and install the necessary split(s)

At the moment, this only works for instant apps. However, the
implementation is generic and could be applied to any application.

Bug: 25119046
Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.EphemeralTest
Change-Id: I6598abb34becfd049fc03199813226736e5057b1