android: Cleanup stack trace symbolizer

This CL performs a little cleanup of the content of
third_party/android_platform/development/scripts/:

- Add ToolPath() and GetAaptPath() methods to
  pylib.constants.host_paths, and remove the old
  versions from symbol.py, adjusting all callers.

  Note that ToolPath() now takes a second argument
  to specify the target CPU architecture, instead of
  relying on a hidden global variable.

  + Add new unit-test (host_paths_unittest.py).

- Remove a few unused global variables in symbol.py,
  and document the few remaining one (especially that
  they are liberally modified from elsewhere!).

  Also make a few functions private by adding an
  underscore prefix to them.

  Remove Uname() which was completely unused.

  Remove FindToolchain() since its result was
  completely ignored.

  Add an optional cpu_arch parameter to
  SymbolInformationForSet(), and make asan_symbolize.py,
  its only user, pass a corresponding argument.

- Cleanup asan_symbolize.py a little, and add an
  optional command-line argument --arch=ARCH to
  specify the target CPU architecture (default
  is 'arm', as in the previous implementation).

- Remove CallCppFilt() from symbol.py and rewrite it
  directly into tools/cygprofile/symbol_extractor.py,
  its only user. Also run the unit tests correctly.

BUG=NONE
R=agrieve@chromium.org,lizeb@chromium.org,egeunis@chromium.org

Change-Id: If172662461a2047d7485eedd4658149283fd1ca7
Reviewed-on: https://chromium-review.googlesource.com/1004635
Reviewed-by: agrieve <agrieve@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Commit-Queue: David Turner <digit@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#549836}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 749445f007a6a34e2b3dab41c3273058a80323c5