Limit number of window context without any window

This change is to prevent misuse of window context from app
and leads to performance drop on system by limit the numer of window
context an app can use. Code snippet below is a sample to cause
this issue:
```
Rect getBounds() {
    Context windowContext = context.createWindowContext(...);
    return windowContext.getSystemService(WindowManager.class)
            .getCuttentWindowMetrics().getBounds()
}
```
This method could be invoked dozens of times and produce dozens of window
tokens. It would slow down the speed of window traversalling. These
token won't be removed until system server has been GC'd.

Test: atest WindowContextTests WindowContextPolicyTests
fixes: 152934797
Bug: 153369119

Change-Id: I927e85a45c05c4d90b51a624ea408ff3a3ffce93