Don't copy apns-conf.xml for apps-only build

http://b/issue?id=2796939
This can fix commands mm and mmm in an unbundled branch.
The cause is that mm & mmm depends on phony target "files", which includes apns-conf.xml.

Change-Id: I72ce1c7c5ddfdcd3bbea5cb9645ca3a48ff8ac12