Allow Soong to export multiple proguard flags files

Soong uses lists of proguard flags files from dependencies instead
of a single concatenated file.  Allow Soong to pass the list to
make, which will concatenate it.

Move the check inside ifdef LOCAL_SOONG_RESOURCE_EXPORT_PACKAGE
in case Soong exports an Android library with an empty
LOCAL_SOONG_EXPORT_PROGUARD_FLAGS.

Bug: 73724997
Test: m checkbuild
Change-Id: Ia8f229ec5f9fc99a23bd471dfa70c9de7316de94