Clean up NullWebViewUtils and test activities.

Most methods of NullWebViewUtils are used to implement an
exception-catching pattern that's error-prone and hard to understand,
which predates the introduction of a WebView feature flag. The different
WebView test activities in CTS have also drifted out of sync with each
other over time.

Clean this up with the following changes:

- Remove all methods of NullWebView except for isWebViewAvailable, and
  reimplement that just as a check for the feature flag.

- Guard creation of WebViews in test activities with isWebViewAvailable
  instead of trying to catch the exception when creation fails. Failure
  to create the WebView will now always result in a simple crash of the
  test on devices which claim to have the feature, which is easier to
  debug and understand than relying on storing and rethrowing the
  exception later.

- Make sure the different activities all detach and destroy their
  WebView correctly.

- Set the WebView's size to match_parent in all layouts using it as
  wrap_contents is inadvisable.

Bug: 147616868
Test: atest CtsWebkitTestCases CtsHostsideWebViewTests \
  EphemeralTest#testWebViewLoads \
  android.text.cts.EmojiTest#testEmojiGlyphWebView

Change-Id: Ieca3ee77639104f2b1ad405290f1d1808bdd835a