Update the namespace for the compute bounds flag.

Previously I created a new namespace based on the filegroup name that
was already defined in the Android.bp. The namespace core_graphics is
already managed and this flag should just join our managed flags
already. This will be better than managing a new namespace just for this
flag.

Bug: 304478551
Test: make
Change-Id: Id116fa94da627a9687992d23cd31820cccf15c6a