Mention `standalone-apex-files` in the Runtime APEX build rules.

Until ART testing and benchmarking make use of the Runtime APEX, we
have to create copies of some libraries under `/system` within the
device environment. Add a note about this in the Runtime APEX build
rules as a reminder.

Test: n/a
Bug: 121117762
Change-Id: I7d849193a782506fd9d5761b6835f72816065940
1 file changed