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