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
Merged-In: I5954fde05256e00a8e01c114ad522f50ed0cfa9f
Change-Id: I5954fde05256e00a8e01c114ad522f50ed0cfa9f
5c8434d1c7