Add power efficiency hint for power hint session.

Background threads sometimes can be very CPU intensive but not critical
from a user's perspective or senstive to a target deadline. Such
workload often time consumes some extra power with no user benefit. This
newly added power efficiency hint allows client to express threads
within the power hint session should be applied with a
power-effiecient-first scheduling strategy, and hence allows the power
implementation to apply such strategy confidently.

Bug: b/244216750
Test: atest VtsHalPowerTargetTest
Change-Id: Iac10b090b208b37f0b7b18a34a45f2da8ecc419b