add default path to flag bp files in mainline module

Mainline path cannot be list if the build is user or if the test doesn't
have the root permission. This change adds a default list for the flag
bp files in mainline module if the caller doesn't have the right
permission.

Test: presbumit
Bug: 365157972
Change-Id: I1ba6b4f99d565b7cbc1d49a5cc7b7a65cae2e140