Add include_make_built_files

If `include_make_built_files` is set to the name of a partition, the
make-built files from that partition will be incorperated into this
soong module. This is to ease the transition to soong built filesystems.

If any files are present in both the soong-built file list and the
make-built one, the soong ones will be preferred.

Bug: 329146343
Test: go test
Change-Id: I456b283e1189116e699ed75357cc056f5d217688