Use repeatable timestamps

This moves all of the date references under build/ to using a single
datetime that can be set manually using BUILD_DATETIME.

It also adds an option, OVERRIDE_C_DATE_TIME, that if set to true, will
redefine __DATE__ and __TIME__ for all C/C++ files so that it matches
BUILD_DATETIME.

Bug: 23117013
Change-Id: I7c17a32b794a5adf40b9cd69136fb0ff9f6084ec