Earlier, we changed the threshold to IMP_FG to allow
HPJs to bypass power saving network restrictions but
later we made a change to instead use network capability
for HPJs, so reverting the threshold to BGFS.
Bug: 177641226
Test: atest ./tests/cts/hostside/src/com/android/cts/net/HostsideRestrictBackgroundNetworkTests.java
Ignore-AOSP-First: This depends on an earlier cl which is not in AOSP
Change-Id: I6364ca04254bdbcc3d2081a70babfbd927e0084e