Do not check for messages if Message Center is disabled.

(cherry picked from commit 63c8f02f33673d013e14045dd6183d3375d6e13b)
Put network worker threads back to sleep when no network connection is present.

(cherry picked from commit 456e7d3bed308eade53a774113f42bdb493e9755)
Use Android 3.1 to build SDk, samples, and tests.
Add assets directory to dev app.
Send proper file name when sending attachment. When sending images form Message Center, change mime type to JPEG since we re-encode them.
Add tests to Dev app. Tests send some none-image file types.
Merge branch 'branch_1.6.1' into fix_file_message_name
Use the disable_message_center config setting for FeedbackDialog interaction. Causes thank you dialog to show/hide the View Messages button.
Merge branch 'message_center_disable' into branch_1.6.1
Merge branch 'fix_file_message_name' into branch_1.6.1