Add new "max-sdk-p blacklist".

This will contain greylist entries accessible by apps with a
targetSdkVersion of less than Q. Currently empty, will be populated in
follow up CLs.

For now, these APIs are just merged into the light greylist at build time,
pending support in the runtime for implementing the correct runtime
behavior.

Also fix a bug in sort_api.sh so it deals with empty API lists
correctly.

Bug: 115609023
Test: m
Change-Id: I213874062f393f96d120648a934ae5ad7aba93af