Deprecate WebSettings.LOAD_NORMAL cache mode.

After the switch to the Chromium network stack, this setting is
effectively the same as LOAD_DEFAULT, and having two settings
with the same meaning is confusing.

This change is blocking https://android-git.corp.google.com/g/205883 in CTS

Change-Id: I02cc23477f107e7f4cec37dcbad4f25503b5e840