Add very basic support for symboled debugging on android

Add very basic support for symboled debugging on android

This only barely works.  We pull down system libraries
once before we attach.  The libraries we pull from
the device do not have debug symbols, but have enough
that we have decent callstacks.  We launch a background
process to repeatedly update a cache directory with
symlinks into our build directory corresponding to the cache
names used on the device, however gdb doesn't watch
the solib-search-path directories to notice the links
as we add them.

Better solutions would be to add support for pulling
down full android symboled system images and using those
instead of pulling libraries off the device as well as
figure out how to get android binaries to support
build-id so that we can present a directory of build-id
associated libraries to gdb on boot and have it to build-id
based lookups of libraries instead of our current broken
watch-logs-and-add-symlinks approach.

If you know what you're doing with this you can actually
make debugging work on the device.  It's not particularly
user friendly yet, but we'll work on it.

I added a build_dir member to skypy.paths.Paths
as a temporary measure before we move off of skypy's
paths and onto mopy's paths.  This helped eliminate
problems with using a relative path in args.build_dir
as is common.

R=abarth@chromium.org, ojan@chromium.org, qsr@chromium.org
BUG=

Committed: https://chromium.googlesource.com/external/mojo/+/f0cdc3145a1945285afead7afb2cfe24391bf725

[abarth-chromium, 2015-01-16 21:17:20]

lgtm

https://codereview.chromium.org/855663003/diff/1/sky/tools/android_library_ca...
File sky/tools/android_library_cacher.py (right):

https://codereview.chromium.org/855663003/diff/1/sky/tools/android_library_ca...
sky/tools/android_library_cacher.py:28: library_regexp =
re.compile(r'(?P<library_path>/system/.*\.so)')
lib?

[eseidel, 2015-01-16 21:39:24]

Committed patchset #1 (id:1) manually as
f0cdc3145a1945285afead7afb2cfe24391bf725 (presubmit successful).