Clean up CarrierPrivilegesTracker.

This change adds several clarifying comments to CarrierPrivilegesTracker
fields and method calls. This change also adds short-circuiting logic
for edge cases to improve readability.

Bug: 147391402
Test: Tested on crosshatch build
Change-Id: I1283078dae6b3bd4e0fe999f15e987f26cc43a8e
Merged-In: I1283078dae6b3bd4e0fe999f15e987f26cc43a8e
(cherry picked from commit 065075b1)