Soong module for selinux files including board-specific directories.

se_filegroup is used to export selinux files from board-specific
directories (e.g. device/google/wahoo/sepolicy).

Use se_filegroup module to export partner extension of compatibility
mapping to build logic in system/sepolicy.

Bug: 33691272
Bug: 74669108
Test: .cil mapping files can be correctly added from vendor directory.
Change-Id: Iaa2a95d0e326cb03a478fc12c1a14ba40e57e117