example/android: fix UiBench workload example

 - There is no longer a get method for Workload, use getInstance
   instead.
 - All pre-/post-workload setup is now done in the UiBench run
   method (start and stop tracing, unlocking, setting brightness, etc.).
   There is no need to have them in our experiment function.

Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>