misc/android: rework GOROOT installation

- Fall back to 'go env GOROOT' to locate GOROOT if runtime.GOROOT() is
  empty (as may be the case if the tool is built with -trimpath).

- Copy all of $GOROOT/android_$GOARCH/bin, not just cmd/go, to
  $GOROOT/bin.

- For consistency with CL 404134, place $GOROOT/bin at the beginning
  of $PATH, not the end.

- Don't use the install target for the "runtime" package to locate pkg/tool.
  As of Go 1.20 "runtime" doesn't have an install directory anyway.
  Since the real reason we need pkg/tool is for commands in "cmd",
  use an arbitrary command (namely "cmd/compile") to locate it.

- Use 'go list' to determine the package import path for the current
  directory, instead of assuming that it is within GOROOT or GOPATH.
  (That assumption does not hold in module mode.)

Updates #58775.

Change-Id: If76ff22bce76d05175c40678230f046a4aff0940
Reviewed-on: https://go-review.googlesource.com/c/go/+/472096
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Changkun Ou <mail@changkun.de>
Auto-Submit: Bryan Mills <bcmills@google.com>

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