Use Proguard -systemjars instead of -libraryjars

Proguard -systemjars will correctly use the system libraries version
of a duplicate class rather than the program's version.

If USE_R8=true then use -libraryjars as R8's proguard emulation layer
does not support -systemjars and it is unnecessary because R8 does not
fail like proguard does when it comes across duplicate classes.

Bug: 30188076
Bug: 69156675
Test: make checkbuild
Change-Id: I99f0cb7a816ce34e5fa209803b7c538fd931d8a7