[WebView] Clean up code for finding paths and sizes of native libs

Now that we no longer pass the paths of the native WebView libraries to
the native loading code we can simplify the code for fetching the path
and the size of the native WebView library.

Also add automated tests (WebViewLoadingTests) for loading native
libraries from test APKs where the libraries are stored either on disk
or as uncompressed libraries in the APKs.

Bug: 28736099
Test: start WebView-app, and ensure that the relro file is loaded into
the app process. Do this for both 32-, and 64-bit apps. And both for a
WebView APK where the libraries are loaded onto disk, and an APK where
the libraries are loaded uncompressed from the APK.
Test: make WebViewLoadingTests && make tradefed-all && \
tradefed.sh run template/local_min --template:map test=WebViewLoadingTests
Change-Id: I00d9b7f0650fe440ebee971eed8482759c7cbc79