Move BUILD_*_FROM_FILE to config.mk

BUILD_NUMBER_FROM_FILE and BUILD_DATETIME_FROM_FILE may need to
be referenced before main.mk has been parsed, move them to config.mk.

Bug: 68865363
Test: m -j TARGET_PRODUCT=full TARGET_BUILD_APPS=Gallery
      aapt dump xmltree out/target/product/generic/obj/APPS/Gallery_intermediates/package.apk AndroidManifest.xml | grep Version
Change-Id: I9361a394469e61fcdf5a2b9509ef2bb2b373d6e7