fix bug with android activity launch on Nexus devices running KitKat(4.4)

Added android:exported="true" to PushHandlerActivity to fix this exception:

04-27 21:50:38.272: W/ActivityManager(574): java.lang.SecurityException: Permission Denial: starting Intent { flg=0x24000000 cmp=com.myawesomeapp.mobile/org.jboss.aerogear.cordova.push.PushHandlerActivity bnds=[64,292][956,420] (has extras) } from null (pid=-1, uid=10099) not exported from uid 10106
fix bug with android not sending cached messages

Upon launching the app from a push notification, messages were not being sent.  Also, removed unused import.
Revert "fix bug with android activity launch on Nexus devices running KitKat(4.4)"

This reverts commit df5a6050d3ae17966a52073186f60fc33866d5ef.
Revert 857b438..2991d6c

This rolls back to commit 857b438be1330d3f00cd05aa206bb47b0051b558.