Narrowing the set of Media URI exemptions for jobs

Only jobs watching for changes to EXTERNAL_CONTENT_URI for Images and
Vidoes will get exemption for jobs.
MediaScanner will scan and insert an entry under each of these URIs
for respective file types.

Test: atest com.android.server.job.controllers.JobStatusTest

Bug: 148887841
Change-Id: I6f9a07d2c0f06c002929056891187f22a6e87e21