[pm] optimize nullableDataDir() flag reading

Attempting to solve native crashes that seem to be caused by this flag reading. This flag is only needed if the app is uninstalled on one user in a multi-user scenario. Optimize the code so that we don't need to check this flag in most of the cases.

Change-Id: I85231efe88d8799e07df2a7339e7740c3a5bac0c
BUG: 307383781
Test: presubmit