Return non-negetive value in getMobileStats method

The current implementation of getMobileRxBytes and all the similiar
method adds up the return values for multiple calls to getRxBytes so if
all of them return UNSUPPORTED for any reason, getMobileRxBytes() would
return a value such as -3. This behavior is not compliance with the cts
TrafficStatsTest which always assume getMobileRxBytes to return a
non-negetive value. The method now will check tha stats get from
getRxBytes method and add them up only if the stats is valid.

Bug: 72473294
Test: run cts -m CtsNetTestCases -t android.net.cts.TrafficStatsTest
Change-Id: I656970ebc8f6506cf17c4353ad46c0178bb65cfd