Add an API to fetch the WebView thread.

Some WebView APIs contain checks ensuring apps only use WebView APIs
from one single thread. The WebView Support Library should perform
similar thread checks. To do so we publish an API to get the WebView
thread, so that the support library knows which thread to check against.

Test: Ran the corresponding CTS tests.
Bug: 38220806
Change-Id: Ibab878e211d9594df6d39f99ce130e28fac7f019