Making context snapshot successfully preload on Android

Before, on Android, the context snapshot never successfully used the
preloaded file. Instead, it always fell back to loading via Java's
AssetManager. See http://crbug.com/421165374#comment2 for details.

Now, we preload correctly, and only preload for the bitness we're on.
This also removes the preload of the wrong bitness non-context snapshot,
which was just an extra warning in logcat.

Additionally, to reduce our footguns, I've moved the keys behind ifdefs,
and only define the keys that should be used on a given platform.

Bug: 421165374
Change-Id: I3c35db399023a656783002dc6bf7348046bc3aff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6602174
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Auto-Submit: Sam Maier <smaier@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1467461}