Add a build_datetime_file arg to build_image.py

If this file exists, build_image.py will use the timestamp from that
file as the mtime of the different indoes in the output img files.

build_image.py already supports hermetic img files (e.g. via
use_fixed_timestamp / timesatmp in the property file), but this feature
is useful since it allows build_image.py to directly use the
build_date.txt file created by android builds.

Bug: 383357338
Test: lunch aosp_cf_x86_64_phone-trunk_staging-userdebug
- export BUILD_DATETIME=10
- commented out installed_*_notice_xml_gz in build/make/core/Makefile
Test: verified that the following .img files are bit identical in top of
CL stack
odm,system_dlkm,vendor_dlkm,odm_dlkm

Change-Id: I1cba5f426268d3a60de709ea4cd8bc154022bdf9