Fix fetch UserAgent crash

[hearsilent]

Catch AndroidRuntimeException that could be raised by the WebSettings.getDefaultUserAgent() in some custom ROMs (e.g. OPPO, HTC, Sony, ASUS, Xiaomi ...).

Crashlytics Issue Details Screenshot:

Crash StackTrace:

Fatal Exception: android.util.AndroidRuntimeException: java.lang.RuntimeException: Invalid reflection
       at android.webkit.WebViewFactory.getProvider(WebViewFactory.java:146)
       at android.webkit.WebSettings.getDefaultUserAgent(WebSettings.java:1244)
       at com.mopub.network.Networking.getUserAgent(Networking.java:133)
       at com.mopub.network.Networking.getRequestQueue(Networking.java:63)
       at com.mopub.nativeads.MoPubNative.requestNativeAd(MoPubNative.java:192)
       at com.mopub.nativeads.MoPubNative.loadNativeAd(MoPubNative.java:177)
       at com.mopub.nativeads.MoPubNative.makeRequest(MoPubNative.java:152)
       at com.mopub.nativeads.MoPubNative.makeRequest(MoPubNative.java:137)
       ...

[louisom]

LGTM

[diousk]

Hi MoPub SDK team,
I've seen this issue in our app again in recent days (about "500 crashes" per day last week),
and I noticed that the "Android System WebView" app on the Google Play also got an update last weeek
(https://play.google.com/store/apps/details?id=com.google.android.webview)

Is this issue relevant to the webview update? and when is this PR ready to be merged?

Thanks.

[hearsilent]

MoPub Team has fixed this issue (328dcd8)
But doesn't merge my PR :'(