Background ANR when calling TracksClient.flush()

[khaykov]

We get a bunch of issues like this:

io.sentry.android.core.ApplicationNotResponding: Background ANR
    at com.automattic.android.tracks.TracksClient.flush(TracksClient.java:368)
    at com.automattic.android.tracks.TracksClient$6.run(TracksClient.java:409)
    at android.os.Handler.handleCallback(Handler.java:942)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loopOnce(Looper.java:201)
    at android.os.Looper.loop(Looper.java:288)
    at android.app.ActivityThread.main(ActivityThread.java:7950)
    at java.lang.reflect.Method.invoke
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:936)