AAPT2: Implement density stripping and initial Split support

When a preferred density is supplied, the closest matching densities
will be selected, the rest stripped from the APK.

Split support will be enabled in a later CL. Command line support is still
needed, but the foundation is ready.

Bug:25958912
Change-Id: I56d599806b4ec4ffa24e17aad48d47130ca05c08