Clean up the JetPlayer class definition

Make the following changes:
- Declare constructors 'explicit' as recommended in go/totw/142.
- Remove a set-but-not-used variable because the compiler warns about it.
- Remove the default JetPlayer constructor because defining a default
  constructor contradicts the comment that says that there is no default
  constructor.

Change-Id: I3b6addedc78e1efbced3e43de5c75cafd9ad24fc
Signed-off-by: Bart Van Assche <bvanassche@google.com>