Do not write SharedPreferences to disk in the main thread

SettingsStore used to call editor.commit() to apply the value of
a setting and write it to disk. That should not be done in the main
thread though as it's a blocking operation. It's better to use
apply() which atomically modifies the SharedPreferences object in
memory and then uses a background thread to write it to disk.

It's safe to replace commit() by apply() as long as the return
value is not used, which is the case.

This fixes reports like:
D  StrictMode policy violation; ~duration=3 ms: android.os.strictmode.DiskWriteViolation
at android.os.StrictMode$AndroidBlockGuardPolicy.onWriteToDisk(StrictMode.java:1615)
at libcore.io.BlockGuardOs.write(BlockGuardOs.java:446)
at libcore.io.ForwardingOs.write(ForwardingOs.java:951)
at libcore.io.IoBridge.write(IoBridge.java:649)
at java.io.FileOutputStream.write(FileOutputStream.java:401)
at com.android.internal.util.FastXmlSerializer.flushBytes(FastXmlSerializer.java:253)
at com.android.internal.util.FastXmlSerializer.flush(FastXmlSerializer.java:274)
at com.android.internal.util.FastXmlSerializer.endDocument(FastXmlSerializer.java:219)
at com.android.internal.util.XmlSerializerWrapper.endDocument(XmlSerializerWrapper.java:68)
at com.android.internal.util.XmlUtils.writeMapXml(XmlUtils.java:408)
at android.app.SharedPreferencesImpl.writeToFile(SharedPreferencesImpl.java:803)
at android.app.SharedPreferencesImpl.access$900(SharedPreferencesImpl.java:59)
at android.app.SharedPreferencesImpl$2.run(SharedPreferencesImpl.java:672)
at android.app.SharedPreferencesImpl.enqueueDiskWrite(SharedPreferencesImpl.java:691)
at android.app.SharedPreferencesImpl.access$100(SharedPreferencesImpl.java:59)
at android.app.SharedPreferencesImpl$EditorImpl.commit(SharedPreferencesImpl.java:604)
at com.igalia.wolvic.browser.SettingsStore.resetCrashRestartCount(SettingsStore.java:766)
at com.igalia.wolvic.VRBrowserActivity.lambda$onCreate$0$com-igalia-wolvic-VRBrowserActivity(VRBrowserActivity.java:296)
at com.igalia.wolvic.VRBrowserActivity$$ExternalSyntheticLambda3.run(D8$$SyntheticClass:0)
at android.os.Handler.handleCallback(Handler.java:938)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loopOnce(Looper.java:214)
at android.os.Looper.loop(Looper.java:304)
at android.app.ActivityThread.main(ActivityThread.java:7918)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1010)