Give CTS a way to force-poll network stats.

Collecting network statistics is pretty heavy, which is why we're
throttling callers.  However, to keep CTS running fast, we provide a
way for tests to force a poll event, instead of making them wait for
the throttle timeout.

Bug: 77908520
Test: atest cts/tests/tests/app.usage/src/android/app/usage/cts/NetworkUsageStatsTest.java
Change-Id: Ia792f0cd495023366ff8c4839df54e7da2ae8331