Do not check for messages if Message Center is disabled.

(cherry picked from commit 63c8f02f33673d013e14045dd6183d3375d6e13b)
(cherry picked from commit f0822ffd143b214fc0d3e99e4f5f7c2365a47252)
Put network worker threads back to sleep when no network connection is present.

(cherry picked from commit 456e7d3bed308eade53a774113f42bdb493e9755)
(cherry picked from commit c474bd167c48582a0e6c1650075c7f31c68faecb)
Use Android 3.1 to build SDk, samples, and tests.

(cherry picked from commit 823950cd10f5ce58bfb737530f6b19eaa411c049)
Merge branch 'branch_1.6.1' into fix_file_message_name