android,uv: relax number of cpus assumption

Relaxes the assumption that number of CPUs will remain the same
during a uv_cpu_info call on Android. Some devices have a power
saving strategy of taking cores offline during execution. In those
cases, uv_cpu_info may return zero values for properties it couldn't
read.
Adds notes to the os.cpus documentation about the mobile quirks.

deps/uv/src/unix/linux-core.c

@@ -798,7 +798,11 @@ static int read_times(FILE* statfile_fp,
     ts.irq  = clock_ticks * irq;
     ci[num++].cpu_times = ts;
   }
+
+/* On Android, the number of cores may change during run time. */
+#ifndef __ANDROID__
   assert(num == numcpus);
+#endif
 
   return 0;
 }

doc/api/os.md

@@ -170,6 +170,13 @@ For example:
 *Note*: Because `nice` values are UNIX-specific, on Windows the `nice` values
 of all processors are always 0.
 
+*Note*: On iOS, CPU speed values are always 0.
+
+*Note*: On Android, returns undefined since Android 8.0. On earlier Android
+versions the CPU values can be inconsistent, since some devices can turn CPU
+cores on and off as an energy saving strategy. Properties can be returned as
+zero for cores that have been turned off while getting them.
+
 ## os.endianness()
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