utils/android/workload: Allow packageless workload
utils/android/workload: Allow packageless workload

The getInstance() class method would look for the requested class'
package on the target device. This commit allows workload classes
to declare their package as an empty string, so that no package
is required for the workload to run.

ARM-software/lisa,credp/lisa,credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,arnoldlu/lisa,bjackman/lisa,mdigiorgio/lisa,bjackman/lisa,ARM-software/lisa,mdigiorgio/lisa,arnoldlu/lisa