Remove biometric chained combine operators

Chained combine operators unnecessarily begin
extra coroutines. It's better to combine
the flows at one time rather than
chain them.

Test: device builds
Test: atest DeviceEntryFaceAuthRepositoryTest
Flag: EXEMPT bugfix
Fixes: 396736084
Change-Id: I334f65a36fa588befe34460a2c1fba38164065b6