Add SystemUI-tests-utils filegroup to reuse in downstream

Because SystemUI-tests android_library conatins AndroidManifest.xml,
it is diffecult to use test utility classes in SystemUITests from
downstream, such as ArcSystemUITests. Having a separate filegroup
module may help to address such difficulty.

Bug: 140464398
Test: make -j SystemUI SystemUITests
Change-Id: Idfc65a3afb30e58efee82a2eccbdbc2b26297364