Make the onRequestRecommendation() method async.

Converted the NetworkRecommendationProvider.onRequestRecommendation()
method into an async call to give implementors more flexibility.

Added unit tests for NetworkRecommendationProvider.

Test: Added NetworkRecommendationProviderTest.
BUG: 32909424
Change-Id: I8c1d2e11baa539744d77a012e85351f750523e3b
Merged-In: Iebe72f260133e9ad1946b0b75e2f69635e154ef3