Update javadoc for executor accepted for VQDS

The callbacks can never be called simutaneously - dev better use
sequential executors.

Test: atest android.voiceinteraction.cts.VisualQueryDetectionServiceBasicTest
Bug: 321997330
Change-Id: Ia46b98ab4c6b6f8393d8aac66a951cd96138d175
2 files changed