Have 'adb' as the default value for when adb_path is not specified

Have 'adb' as the default value for when adb_path is not specified.

This prevents an AttributeError in AndroidShell when no adb_path is specified on the command line.

Fixes domokit/devtools#33

R=ppi@chromium.org, ppi

Committed: https://chromium.googlesource.com/external/mojo/+/a6b862ed82a1a1d34d1f073e942e6a6964baf696

[ppi, 2015-09-01 12:08:34]

There's gotcha here - command line option beats any defaults inferred later, so
we'd use the adb the user has in path over the one proper for the checkout,
causing hard to debug issues if the versions are different.

Instead, we should apply the adb default as late as possible - after the
"inferred paths" magic for Chromium checkout fails to find adb in the expected
location.

[etiennej, 2015-09-01 12:26:39]

On 2015/09/01 12:08:34, ppi wrote:
> There's gotcha here - command line option beats any defaults inferred later,
so
> we'd use the adb the user has in path over the one proper for the checkout,
> causing hard to debug issues if the versions are different.
> 
> Instead, we should apply the adb default as late as possible - after the
> "inferred paths" magic for Chromium checkout fails to find adb in the expected
> location.

Done, per offline discussion.

[ppi, 2015-09-01 12:28:09]

lgtm

[etiennej, 2015-09-01 12:28:33]

Committed patchset #2 (id:20001) manually as
a6b862ed82a1a1d34d1f073e942e6a6964baf696 (presubmit successful).