Support loading a stub WebView using a donor package.

Support loading a WebView package which specifies the name of a "donor"
that provides missing files. This allows a preinstalled stub WebView to
function by loading its code and assets from the preinstalled Monochrome
implementation, as long as the versions are close enough that the
manifest contents are compatible, which should be fine since
preinstalled versions will match.

To do this, we replace the stub's code paths in AppplicationInfo with
the donor's, so that all Java and native code and resources are loaded
from the donor APK at runtime instead of from the (mostly empty) stub.

To get the ClassLoader with the modified path cached as if it was the
regular path, we introduce a new "cacheKey" parameter in
ApplicationLoaders.

Bug: 21643067
Test: build "new" stub WebView upstream in chromium and test loading
Change-Id: I08cc9122b1c9def3e1206974f3e0e8973cca3419