Ignore summary time if it has children

We don't show the summary time if there are children, and even when the
time of them is explicitly set, it's often wrong.

Test: RankingHelperTest
Bug: 330193582
Fixes: 334945760
Flag: android.app.sort_section_by_time STAGING
Change-Id: I720fdb65ad5bd2bad1532f44f54780264b529b85