For an R2 Passpoint AP, there might be zero or more OSU providers
associated it. Add an API to retrieve the associated OSU providers.
Bug: 62235301
Test: manual test by exercising this API in WifiTracker and verify
the content of the OSU provider
Change-Id: I2e730f0b16d80e0b97b4ce7cbd086b606e71eb0c
Merged-In: I2e730f0b16d80e0b97b4ce7cbd086b606e71eb0c