Reland "jni_zero: Always multiplexing when GN arg is set"

This is a reland of commit 90703dea1b7688b69dfcf907acf53e777ee52b76

This was reverted because it was failing on release bots with
skip_secondary_abi_for_cq. I reproed and fixed - see PS#1->2

Original change's description:
> jni_zero: Always multiplexing when GN arg is set
>
> We had the ability to set multiplexing off on a per-target level, but we
> didn't use it, and may as well clean up unused confusing stuff if it's
> providing no value.
>
> Bug: 40612925
> Change-Id: I7f4f214c55453b1762d360466031cc787862f1a5
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5631659
> Commit-Queue: Andrew Grieve <agrieve@chromium.org>
> Auto-Submit: Sam Maier <smaier@chromium.org>
> Reviewed-by: Andrew Grieve <agrieve@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1315002}

Bug: 40612925
Change-Id: Iaad555adb69c55f4bec91c048150605b0e02c3cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5631528
Auto-Submit: Sam Maier <smaier@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1316620}

• build/config/android/linker_version_script.gni[diff]
• build/config/android/rules.gni[diff]
• build/toolchain/android/BUILD.gn[diff]
• chrome/android/modules/chrome_feature_module_tmpl.gni[diff]
• third_party/jni_zero/jni_zero.gni[diff]
• third_party/jni_zero/sample/BUILD.gn[diff]