Add skeleton code for Treble labeling test

Treble labeling test is a SELinux test ensuring that platform partitions
and vendor partitions are correctly split. The test uses the following
as inputs:
* preinstalled apps
* seapp_contexts files
* file_contexts files
* A precompiled SEPolicy binary

This change adds skeleton code for the Treble labeling test, and a phony
target named check-selinux-treble-labeling to run the test manually.

Bug: 406185315
Test: m check-selinux-treble-labeling
Change-Id: I3afbf715f2865ae9b985b97835af2a7fd1fb6349