Adds caller check to getAllPackages()

This change enforces that only system, root or shell may call
getAllPackages(), a hidden API that shares all package names regardless
of user, instant app or package visibility rules.

Bug: 174661955
Test: 'adb shell pm compile -a -m speed' doesn't fail with
      SecurityException
Change-Id: I86a03c46896f21cdbeeda2af150675fe77d61955