Add and use ServiceProvider interface on fingerprint2.1

This interface should allow easy support for multiple HAL interfaces.
For example,

for (ServiceProvider p : providers) {
    if (p.containsSensor(sensorId)) {
        p.authenticate(sensorId, ...)
    }
}

or,

ServiceProvider provider = getProviderFor(sensorId)
if (provider != null) provider.authenticate(sensorId, ...)

This should reduce lots of repeated null checks, sensorId checks,
etc. and give FingerprintService a easy way to perform operations
on providers, given a sensorId.

Some of the FingerprintManager code is not sensorId-specific,
but we can update those in the future. Otherwise we have to
touch multiple projects (settings, etc). For now, all client-side
code assumes only a single sensor, so that's what we'll support
from FingerprintService.

Bug: 168843828
Test: No effect on existing devices
Test: atest com.android.server.biometrics
Test: atest com.android.systemui.biometrics

Change-Id: Icca5bc1e128afa1bb7b2c05227e87567e564e4cf