Update removeAll test path

1) for the old HIDL interfaces, remove(0) means to remove all
   enrollments. Thus, have the test HAL pass back the fake enrollmentIds
   instead of just "0"
2) RemovalClient never receives null identifiers. Update it to
   be @NonNull and remove the unnecessary null check

Test: atest CtsBiometricsTestCases
Bug: 183755255
Change-Id: I2cffc4afb77c009ef75ad6ebae358e7779917744