API review changes to CallDiagnosticService

1. Remove NETWORK_TYPE and AUDIO_CODEC constants in favor of existing
constants in TelephonyManager and Connection.  Add API docs explaining
the simplifications of these two concepts.
2. Add more documentation on DiagnosticCall#sendDeviceToDeviceMessage
to explain the D2D protocol and the bandwidth challenges inherent.
3. Remove DiagnosticCall#getCallDetails.
4. Provide provision for developer-defined Executor in the
CallDiagnosticService; added docs there and in DiagnosticCall explaining
the threading model used.

Test: Ran CTS tests
Fixes: 182241145
Change-Id: Icb0d8a05add778a0cfcbe3448063cd9dcee3abc1