Add throttling to generateDisplayHash

Calling generateDisplayHash can cause the system to slow down if there
are too many requests. Add throttling logic to prevent apps from calling
the API too frequently. The throttling time is based on a value sent
from ExtServices so it can be adjusted later

Test: DisplayHashManagerTest
Bug: 155825630
Change-Id: Id49d1c4113fc65d7b73a1e7b9fc73f50a8d7041f