Move MTE mode settings to a product variable.

Allow product configuration of memtag target list by
moving the current set into a product variable instead of the various
.bp files.

The default list of memtag targets can be found in
  build/make/target/product/memtag-common.mk

This is NFC as all targets in the list already have "memtag_heap: true"
in the build files.

Bug: 280343521
Test: no functional change
Change-Id: I5954fde05256e00a8e01c114ad522f50ed0cfa9f