Leak in SystemJobService

Hi everyone,

we're using WorkManager in an app in which we've also [LeakCanary](https://square.github.io/leakcanary/) configured and we always receive this warning after a run of each Periodic Worker:

```
16:49:08.888 LeakCanary ​
====================================
HEAP ANALYSIS RESULT
====================================
1 APPLICATION LEAKS

References underlined with "~~~" are likely causes.
Learn more at https://squ.re/leaks.

774 bytes retained by leaking objects
Signature: 6491376cfe66b1cac0d47b3ae180fd902719aa5
┬───
│ GC Root: Global variable in native code
│
├─ android.app.job.JobService$1 instance
│ Leaking: UNKNOWN
│ Retaining 802 B in 15 objects
│ Anonymous subclass of android.app.job.IJobService$Stub
│ this$0 instance of androidx.work.impl.background.systemjob.SystemJobService
│ ↓ JobService$1.this$0
│ ~~~~~~
╰→ androidx.work.impl.background.systemjob.SystemJobService instance
​ Leaking: YES (ObjectWatcher was watching this because androidx.work.impl.background.systemjob.SystemJobService
​ received Service#onDestroy() callback)
​ Retaining 774 B in 14 objects
​ key = 155d4c85-1073-4df1-bd0f-250b40c959c8
​ watchDurationMillis = 5248
​ retainedDurationMillis = 246
​ mApplication instance of xx.yy.MyApp
​ mBase instance of android.app.ContextImpl
====================================
0 LIBRARY LEAKS

A Library Leak is a leak caused by a known bug in 3rd party code that you do not have control over.
See https://square.github.io/leakcanary/fundamentals-how-leakcanary-works/#4-categorizing-leaks
====================================
0 UNREACHABLE OBJECTS

An unreachable object is still in memory but LeakCanary could not find a strong reference path
from GC roots.
====================================
METADATA

Please include this in bug reports and Stack Overflow questions.

Build.VERSION.SDK_INT: 23
Build.MANUFACTURER: alps
LeakCanary version: 2.6
App process name: xx.yy.MyApp
Stats: LruCache[maxSize=3000,hits=101,misses=300675,hitRate=0%]
RandomAccess[bytes=11383030,reads=300675,travel=696095628841,range=23369842,size=28770980]
Heap dump reason: 1 retained objects, app is not visible
Analysis duration: 202141 ms
Heap dump file path: /storage/emulated/0/Download/leakcanary-xx.yy.MyApp/2021-03-08_16-45-32_740.
hprof
Heap dump timestamp: 1615218548838
Heap dump duration: 13889 ms
====================================
```

WorkManager library in use: `androidx.work:work-rxjava2:2.5.0`
LeakCanary library in use: `com.squareup.leakcanary:leakcanary-android:2.6`

Any advice? Did you see this warning yet?

I don't know if can be a useful info, but the app we're developing is a system app used on embedded device (Android based, obviously)

先从 issue 中的 LeakCanary trace 和 Android API 23 上的 SystemJobService 生命周期入手，然后使用 WorkManager 2.5.0 和 LeakCanary 2.6 配合 periodic worker 进行复现。完成的标准是确定被保留的 service 是否为可复现的 library leak，并记录或修复结果；issue 未指定源文件或测试。

由索引模型根据 Issue 内容生成。