SDK / exoplayer

- Assertion chaining is generally discouraged. For example, because it's harder
  to determine which assertion failed given a line number.
- Also removed chaining of the form obj.actionX().assertY(), because it's easy
  for someone editing the test to accidentally delete actionX() when deleting
  assertY(), where-as actionX() may often be important for subsequent assertions.

PiperOrigin-RevId: 308820503