Expose PLATFORM_SECURITY_PATCH_TIMESTAMP.

We have use case that uses the timestamp for PLATFORM_SECURITY_PATCH in
device specific Makefile, but need to handle the platform-dependent
`date` command. This CL computes and exposes
PLATFORM_SECURITY_PATCH_TIMESTAMP from the core build system.

Bug: 80311893
Test: Use PLATFORM_SECURITY_PATCH_TIMESTAMP in device-specific folder,
      on both of Linux and macOS. Check the value.
Change-Id: I27a88dcebde7a1c204491f5f5c45c5160398dbb6