Partially revert BUILD_DATETIME change

Change Ibe4ceb1b224c5e8f3e1a8a59cfee89217765a901 caused kati to
regenerate on every build:
$(shell cat out/build_date.txt) was changed, regenerating...
Switch back to $(shell date +%s) which is special cased in kati
to not cause a regeneration.  Everything besides DATE will still
use BUILD_DATETIME_FROM_FILE from soong_ui.

Once BUILD_NUMBER is moved to soong_ui $(DATE) is no longer
necessary and the whole thing can be removed.

Bug: 70351683
Bug: 73352417
Test: m && m doesn't regenerate
Test: m BUILD_DATETIME=1234 shows up in ro.build.version.incremental
Change-Id: I8444beb2b47a49095ef760083fc85f220821bffe