webview: refactor compatibility check.

Refactor the logic used to determine if a WebView implementation APK is
compatible with the current OS version:

- Move the constants defining the minimum target SDK version and class
  name to use to the WebViewFactoryProvider interface, as that's the
  "root" of what the implementation actually implements. This keeps the
  requirements separate from the code responsible for actually loading
  the implementation in order to simplify things for future Mainline
  work.

- Change some of the naming to be more generically about compatibility
  where it makes sense, instead of referring specifically to
  targetSdkVersion, as we don't intend to use targetSdkVersion as the
  gating criteria for future incompatible changes.

- Instead of using the "real" compatibility check in
  WebViewUpdateServiceTest, just have the test system interface accept
  any package as valid by default, and mock the check in the specific
  test where it matters.

Bug: 373617389
Flag: EXEMPT refactor
Test: atest WebViewUpdateServiceTest; atest CtsWebkitTestCases
Change-Id: I4b6dca9ef4fbd29a8f59802ceed489fb296e235e