android: fall back to /proc/cpuinfo when /proc/stat is unreadable

[dvrdnz]

Problem

On Android, /proc/stat is typically unreadable by regular apps (including
Termux) — it's blocked by SELinux policy since Android 8. When this happens,
CpusWrapper::refresh() currently returns early with an empty cpus list,
so no CPU information (name, vendor, brand, frequency) is ever populated,
even though /proc/cpuinfo — which is readable — has all of it.

Fix

Add an Android-only fallback, build_cpus_from_cpuinfo, that builds the CPU
list from /proc/cpuinfo (via the existing get_vendor_id_and_brand) when
opening /proc/stat fails. It's only triggered on the first refresh (first),
matching the existing pattern used elsewhere in this function. CPU frequency
is optionally populated via the existing get_cpu_frequency, which reads
/sys/devices/system/cpu/cpuN/cpufreq/scaling_cur_freq and doesn't depend on
/proc/stat.

Note that cpu_usage() will remain 0.0 for CPUs built this way — usage
percentages require successive /proc/stat snapshots, and there's no public
Android API providing equivalent per-CPU tick counters. This is documented on
the new function.

Testing

Verified on a physical Android device (Termux):

• Before: System::new_all() → cpus() returns 0 CPUs
• After: cpus() returns 8 CPUs, each with correct name (cpu0..cpu7),
  vendor (ARM), brand (e.g. Cortex-A725), and frequency

This was originally surfaced while building
otree on Termux via cargo install,
where vergen's si feature emitted VERGEN_SYSINFO_CPU_VENDOR /
_CPU_BRAND / _CPU_FREQUENCY warnings because sysinfo couldn't resolve
CPU info on this platform.

Companion PR for the same root cause, affecting Users: #1714

[GuillaumeGomez]

Maybe only move/modify this sysinfo_debug if on android since the information is still kinda computed.

[dvrdnz]

Thanks, good point. I updated this so the non-Android path keeps the existing
debug message, while Android now logs that CPU usage information could not be retrieved from /proc/stat.

I also rechecked the behavior on the Android/Termux device:

• /proc/stat is not readable: Permission denied
• /proc/cpuinfo is readable and lists 8 processors
• main returns 0 CPUs from System::new_all()
• with this PR, System::new_all() returns 8 CPUs
• names are cpu0..cpu7, vendor is ARM, brand is Cortex-A725
• frequency is populated
• usage remains 0 as expected, because it still needs /proc/stat snapshots

So the fallback only recovers the static CPU information and frequency; CPU usage remains unavailable on this Android path.