repack_bootimg: replaces the ramdisk file directly

Currently we remove two args `--ramdisk original_ramdisk` from the
output of --format=mkbootimg upon unpack, then append `--ramdisk
new_ramdisk` later.

Refactoring this to replace the original_ramdisk with the new_ramdisk
directly from the output of --format=mkbootimg.

Bug: 174443192
Test: repack_bootimg.py --src_bootimg <> --dst_bootimg <>
Test: unpack_bootimg --boot_img <dst_bootimg> then unpack the ramdisk
Change-Id: I805685409b473a2fea4322b475882125f8c2427c