For #26219
Refactoring the interface between Android service and Android datastore
to use 1 common datastore interface: `fleet.AndroidDatastore`
These changes are based on feedback from the recent Backend Sync.
```mermaid
---
title: Partial class diagram
---
classDiagram
direction LR
class `android.Service`
<<interface>> `android.Service`
class `android/service.Service`
`android/service.Service` ..|> `android.Service`: implements
class `fleet.AndroidDatastore`
<<interface>> `fleet.AndroidDatastore`
class `fleet.Datastore`
<<interface>> `fleet.Datastore`
class `android.Datastore`
<<interface>> `android.Datastore`
`android/service.Service` *-- `fleet.AndroidDatastore`: USES (THIS IS THE KEY CHANGE)
`fleet.Datastore` *-- `fleet.AndroidDatastore`: contains
`mysql.Datastore` ..|> `fleet.Datastore`: implements
`fleet.AndroidDatastore` *-- `android.Datastore`: contains
`mysql.Datastore` *-- `android.Datastore`: contains
`android/mysql.Datastore` ..|> `android.Datastore`: implements
```
#15560
Probably best to review commit by commit.
First commit adds the mockimpl files, second commit amends README.md and
third commit fixes golangci-lint issues.
- [X] Manual QA for all new/changed functionality
Tested by adding a dummy method to service.go and running `make
generate-mock`.
---------
Co-authored-by: Victor Lyuboslavsky <victor.lyuboslavsky@gmail.com>
