Make sure to log reason for any broadcast skip

A few cases were unreported; fix that.  Along the way, consolidate
boilerplate log string construction into a single helper method rather
than duplicating it everywhere.

Fixes: 179301795
Bug: 177729759
Test: atest android.app.activity.BroadcastTest
Test: atest CtsOsTestCases
Change-Id: I18bdfba7e43bcea64feb4c4341189ff0d82a5466