Attempt v2 of "Disables non-system AccessibilityMenu for secondary users."

Re-landing of commit 43cd49fa
but uses createContextAsUser instead of createPackageContextAsUser.
createPackageContextAsUser looks up the package info for non-system
packages, which caused a boottime regression for A11yManagerService
from 5ms to ~40ms. This new version has no regression and functions
the same for our purposes.

Bug: 284196235
Test: Create secondary user, observe only one Accessibility Menu entry
      in Settings > Accessibility.
Test: v2/android-crystalball-eng/asit/perf/boottime_test
      locally and with go/abtd
Change-Id: I8410d818891690d9d3307897b168b94125d41b16