Update enum value in MediaSessionStageChanged

Following the proto best practice, enum value should start from 0.

Test: Manual
Bug: 271419889
Eldar: https://eldar.corp.google.com/assessments/997831806/revisions/2?jsmode=o#sections/999001
Change-Id: I6b7af82f9bd651ce5327e3d381dc02e3764a84e1
1 file changed
tree: fb228b40231f8b714dc97e9ac01a657d420622d3
  1. stats/
  2. OWNERS