Add metrics for software and external source detections.

We add more metrics for software detection and
external source detection.

Android Metrics Design Review : eldar/276723226

Verify the metrics logs manually as below:

Step 1: m statsd_testdrive
Step 2: statsd_testdrive 434
Step 3: run the test
Step 4: We can see the log as below after 1 minutes from Step 2.

hotword_detector_events {
    event: EXTERNAL_SOURCE_DETECTED
}

hotword_detector_events {
    event: ON_DISCONNECTED
}

hotword_detector_events {
    event: START_EXTERNAL_SOURCE_DETECTION
}

Bug: 207717787
Bug: 233704035
Test: build pass
Test: manual. The logs are expected after following test steps
Change-Id: Ib268902d63c580cec9cbd65a3816fd5ba003eaff
(cherry picked from commit 37f8b3ec)