Define a new permission for appsearch isolated storage service

This permission will be used for only allowing platform-signed modules to
call AppSearch isolated_storage_service.

Bug: b/394401673
Flag: com.android.appsearch.flags.enable_isolated_storage
Test: AppSearchSessionCtsTest

Change-Id: I6dc25f2c3fb894fcb6711e0a4a055dbf9cd06cb2