Support regenerating partition table with bpttool in sign_target_files_apks

For Android Things targets (or any other target which has
BOARD_BPT_INPUT_FILES defined), add_img_to_target_files will generate a
partition-table.img using bpttool. It also adds the final combined .bpt
definition file into target-files in IMAGES/partition-table.bpt.

When we're signing using sign_target_files_apks, add_img_to_target_files
needs to regenerate the partition table, but META/misc_info.txt still
contains the original list of bpt input files from the build that aren't
available. This change extracts the final bpt from the input
target-files, adds it to META/ in the output target-files, and then
updates the board_bpt_input_files property to point to it.

Bug: 72837107
Test: Local sign_target_files_apks run of locally built target-files
Change-Id: Id79125208f31c78b1ac2079172f9c91a9203849b