Use classloader namespace instead of lib paths to load WebView libs.

When loading WebView's native libraries we now have a classloader
pointing to the namespace of thise libraries - so we no longer need to
explicitly reference those libraries by their path names.

Bug: 62860565
Test: Start a WebView-using app. Ensure that libwebviewchromium.so is
loaded into the app process.

Change-Id: I205131f4b5fac7c33374560515b85ddef19a7ce9