don't interpret milliseconds as nanoseconds

The bug fix is in the readEvents() wrapper, but also make sure that
both the Event and LogReader interfaces only expose milliseconds.

No test changes, because the Java-layer didn't change, the bug was
in the JNI wrapper, below the level that's easily testable.

Bug: 37205954
Test: runtest --path frameworks/base/core/tests/coretests/src/android/metrics
Change-Id: I0c4ad9233a81bcf585ab525b3a5cc63fbb645093