Use strings to better represent resource values.

APK manifest might or might not contain values for all flags. In this case we don't want API callers to guess the correct defaults.
With this implementation, they can simply assign attribute values to strings, and let the system do the parsing and defaulting.

Bug: 297916136
Test: atest PackageManagerTest
Change-Id: I32a9077f255db1797b066a01d9f1559f55b8919b