os/user: use os.UserHomeDir for user.HomeDir

Using os.UserHomeDir for user.HomeDir helps us deduplicate the
logic and keep the behavior consistent.

Also make os.UserHomeDir return "/sdcard" in android.

See: https://go-review.googlesource.com/c/go/+/37960/1/src/os/user/lookup_stubs.go#48

Fixes #31070

Change-Id: I521bad050bc5761ecc5c0085501374d2cf8e6897
Reviewed-on: https://go-review.googlesource.com/c/go/+/169540


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>