Add junit/textui/** to the proguard filter

Fixes build breakage caused by having duplicate versions of
a class in the application and in the API. This workaround will
be removed once junit classes have been removed from the API.

Bug: 30188076
Test: make Browser2 ANDROID_FORCE_JACK_ENABLED=disabled
Change-Id: I9a9031d3ef6d82f35413b229d588a605f9ed39dc