Use string for CREATE_APP_SPECIFIC_NETWORK permission

This allows using the permission in 25Q4 before it becomes a
public API in 26Q2.

Flag: android.net.platform.flags.create_app_specific_network_permission
Test: TH
Bug: 435313135
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:60f7743f773b05a98e651c953631e93957291bf5
Merged-In: I8d336a588eddf9a027079e208b2b96bb6c163e86
Change-Id: I8d336a588eddf9a027079e208b2b96bb6c163e86
3 files changed