Prefix CDM's "primary" tag with "android.companion"

Prefix CDM's "primary" tag with "android.companion" to bring it in line
with other similar tags used in Android Applications Framework:
- android.service.notification.default_filter_types
- android.view.merge
- android.media.tv.input
etc.

Expand Javadoc for android.companion.CompanionDeviceService

Bug: 198539218
Test: atest CtsCompanionDeviceManagerCoreTestCases
Test: atest CtsCompanionDeviceManagerUiAutomationTestCases
Change-Id: I10fd70c24b6a653dd03e1b74dc682194d867df93