Allow linear backoff policy for UI jobs.

UI jobs were initially required to have an exponential backoff
as a way to reduce keep-alive abuse. Now that UI jobs have a
cumulative execution limit, there's little reason to disallow a
linear backoff policy.

Bug: 261999509
Test: atest CtsJobSchedulerTestCases:JobInfoTest
Change-Id: I88300b0fb64b1ca34123d3e16eb8e1fa81c586d6