Rename BufferRole.frequency -> probability -- HAL.

"Frenquency" often refers to the number of occurrences over a period of
time, while "probability" refers to the number of occurrences of one
event over the number of occurrences of all events. "Probability" is a
better name for this field.

Fixes: 183117895
Test: VtsHalNeuralnetworksTargetTest
Test: NNT_static
Change-Id: Ic86f73b8be2aed567ae4ca17bdb3a57c658fb349