Migrate away from WebView fallback logic.

We no longer need to support fallback packages and need to migrate them
back to being enabled (but only once, to allow the user to disable them
manually later). This CL:

1) Removes the logic which enables/disables packages and the test cases
   which only cover the enabling/disabling logic.

2) Checks at boot time if the fallback logic is enabled; if so, it
   re-enables the fallback package if one exists and then disables the
   fallback logic permanently.

3) Amends remaining test cases to use "primary" and "secondary" instead
   of primary/fallback, to keep testing that higher priority packages
   are more preferred without involving the fallback behaviour.

4) Fixes a bug in the package selection logic that would not have
   triggered in the previous setup (as our production configs never had
   more than one available-by-default, non-fallback packages) that was
   revealed by one of the modified test cases: the boot time logic to
   reset the user's setting if their previously chosen provider was no
   longer available triggered even if the user had not actually chosen a
   provider, causing the automatically selected provider to be
   considered a manual user choice, and preventing a higher priority
   provider from being automatically chosen if it later becomes
   available.

Bug: 126588129
Test: atest WebViewUpdateServiceTest
Change-Id: I532c0faae2427de7132f716725bf62e256975410