Could you tell how to calculate the round trip with the Oboe implementation?
Round trip test environment is according to https://github.com/google/oboe/blob/master/apps/OboeTester/docs/Usage.md
Some of these tests require an audio Loopback Adapter that plugs into a 3.5 mm jack. If the phone does not have a 3.5 mm jack then you can combine that with a USB to 3.5mm adapter.
Is it really needed? I still can get the latency result without loopback adapter in a quiet test environment, is this result wrong?
In my Android phone (Oneplus 8, Android 10, CPU: Qualcomm 865, 8GB+128GB), I got latency.msec = 35.xx.
The loopback adapter is required to test the signal path over a wired connection (either 3.5mm or USB).
The result you have measured is the over-the-air signal path from the built-in speaker to the built-in microphone. This is typically higher than a wired signal path because of extra signal processing introduced to improve the sound quality of the built-in speaker.