Revert NPMS threshold to BFGS.

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
Change-Id: I6c1916ca9e8ff9555bbacda678ce53f8373c8960