Not using app op code constant directly from PackageInstaller

Since AppOpsManager op codes are hidden apis, they can be modified by
OEMs. Referencing the codes directly prohibits partners from freely
using the integer ids to assign to their own op codes. Using a string
identifier to indirectly infer the integer code is more flexible.

Test: Builds, boots, existing tests pass

Bug: 65534401
Change-Id: I5a69690871b5b55f2f8d45d0cf53b5f25ff5abff