apf: Add immediate encoding tests for LI

This tests that ApfGenerator knows how to optimally encode positive and
negative immediates of various sizes.

Equivalent tests will follow for LDDW and STDW.

Change-Id: Ia904aecb155c78569e3cf32a2431570281570481
Bug: 73804303
Test: runtest tests/net/java/android/net/apf/ApfTest.java
(cherry picked from commit 18050000)