Use only the related flag in test setup

Explicitly save and restore the enable_biometric_property_verification
flag in test setup and teardown, without impacting other flags within
the same namespace. This will remove the requirements of adding
unrelated flags into WritableFlags.

Bug: 389790699
Flag: TEST_ONLY
Test: atest BinaryTransparencyServiceTest
Change-Id: I7602ec86c9f8fb90bb77016b09435e060d44cae8