RequiresFlagsDisabled: return false when flag not defined

We are filtering out RO + disabled flags from the aconfig storage system. The infra for the RequiresFlagsDisabled annotation throws an error when a flag is not found in the storage. Instead of failing, treat the flag as false.

Bug: 383122121
Test: m
(cherry picked from https://android-review.googlesource.com/q/commit:da5ddad1ae1c82fffe85eae04bf49115a4167f8b)
Merged-In: Ib7013ebcc6fcb26af1adfd78c4c3b4f73ed99760
Change-Id: Ib7013ebcc6fcb26af1adfd78c4c3b4f73ed99760