Write combined bpt definition directly into META/partition-table.bpt

...instead of copying from IMAGES/partition-table.bpt during
sign_target_files_apks. This addresses a comment on go/oag/665565, but
is done as a separate change so that it can be cherry-picked later (or
not at all) to downstream branches to avoid breaking signing of existing
target-files zips.

Bug: 72837107
Test: Local sign_target_files_apks run of locally built target-files
Change-Id: Id14c859eefe075fc56b15869f2f21c570eb07e65