Build contexts files with Soong

This is to migrate sepolicy Makefiles into Soong. For the first part,
file_contexts, hwservice_contexts, property_contexts, and
service_contexts are migrated. Build-time tests for contexts files are
still in Makefile; they will also be done with Soong after porting the
module sepolicy.

The motivation of migrating is based on generating property_contexts
dynamically: if we were to amend contexts files at build time in the
future, it would be nicer to manage them in Soong. To do that, building
contexts files with Soong can be very helpful.

Bug: 127949646
Bug: 129377144
Test: 1) Build blueline-userdebug, flash, and boot.
Test: 2) Build blueline-userdebug with TARGET_FLATTEN_APEX=true, flash,
and boot.
Test: 3) Build aosp_arm-userdebug.

Change-Id: I49206e656564206d6f7265206361666665696e65