[webview] Annotate API parameters as @Nullable or @NonNull

Annotate WebView API parameters as either @Nullable or @NonNull. When a
method returns a nullable type, add this to the javadoc as well.

Deprecated methods were ignored.

Bug: 119254822
Test: none (only changes annotations, no change in logic)
Change-Id: I701108240fd5905e1085a9e8bcce44567e517892
2 files changed