`availableFrameCount` tracks the number of frames that is avilable on the
`SurfaceTexture`, but haven't been used (by `updateTexImage()`) yet. Thus
semantically this counter should only be decremented after calling
`updateTexImage()`, not before it.
Also reworded `getPendingFrameCount()` javadoc, "external texture" is an
internal state that is not publicised anywhere.
PiperOrigin-RevId: 488765174
