[MS10.1] Move multiplySafeByRational to NetworkStatsUtils

Since NetworkStats related code will be moved to the module.
multiplySafeByRational in the NetworkUtilsInternal cannot be
accessed after that. Thus, create another utils class that
will be moved with NetworkStats code and put the function
into it.

Test: atest NetworkStaticLibTests:com.android.net.moduletests.util.NetworkStatsUtilsTest
Bug: 204830222
Change-Id: I96f3ac02e57b7325ed53988285770f478dee529e