Add tracing for broadcast sending.

Trace broadcasts at the time they're being processed, including
information about the caller and the intent payload.

Bug: 33645477
Test: https://screenshot.googleplex.com/9W1ugzJtVeK &
https://screenshot.googleplex.com/3RbERMPo1sA
Change-Id: I2fcfb8e056881f4ad60ec2ec3c7bf3e0e1060194