Update docs for methods that require subscriberId

In Android 10 additional restrictions were required to access the
subscriberId. The NetworkStatsManager has several methods that accept
a subscriberId of the mobile network for which usage should be queried.
This commit updates the docs for these methods to reference the new
access restrictions and offer null as an option to obtain the usage
for all mobile networks.

Fixes: 157871064
Test: m docs
Change-Id: I95c730c9418fced6312eb3ba4e0d69e6299f3ded