docs: Correct labeling for two USB events

Per bug, these were mislabeled as broadcast actions, when they're
actually activity intents. This has consequences post-O, since
broadcast actions are now subject to limitations.

Doc is staged to:

http://go/dac-stage/reference/android/hardware/usb/UsbManager.html

Bug: 36672976
Test: make ds-docs
Change-Id: I2b515b1ac152de389f952af21166ed2ba7843044