Add -systemjars support to Proguard

Try and address the problem of using Proguard with applications that
duplicate some classes in android.jar, e.g. applications that
statically include junit.

See https://sourceforge.net/p/proguard/discussion/182455/thread/76430d9e/
for more information.

Rebuilt proguard with ant 1.10.1 and gradle 2.1

Bug: 30188076
Bug: 69156675
Test: make checkbuild
Change-Id: I574becf07ed4a247e966f2c023e4b95b5ffc4011