Migrate SearchActivityPreferencesManager to use GURLs.

This change replaces all URL Strings in SAPM with GURL, and embraces
GURL java-side serialization for storing and retrieving cached data.

The mechanism aims to be backward compatible, allowing non-serialized
GURLs to be read directly; this may result in a singular partial
initialization of native libraries.

The change migrates integration tests of SAPM to UnitTests suite, as
these are, quite literally, unit tests in integration suite. While
doing so, we also increase test coverage.

Change-Id: I090af66ec5c8c3426b4f653929ed4d1a67482d38
Fixed: 40241069
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5464499


Commit-Queue: Tomasz Wiszkowski <ender@google.com>
Reviewed-by: Gang Wu <gangwu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1289501}