Make BugreportManager a public API that respects carrier privileges.

This permission model is only allowed for BUGREPORT_MODE_TELEPHONY to
let carrier apps (even from the Play Store / not pre-loaded) access
BugreportManager to trigger connectivity bugreports.

This also requires SELinux policy changes to allow non-system apps to
get an instance through Context#getSystemService, and then dumpstate
also needs permissions to write public apps' files.

Minor documentation and formatting fixes as well.

BYPASS_INCLUSIVE_LANGUAGE_REASON=leaving variable name derived from
"bugreport-whitelisted" sysconfig alone for now

Bug: 161393541
Test: atest CtsCarrierApiTestCases:BugreportManagerTest
Change-Id: I5a38e7a040fa23146fa6c1e785db102b066ad167