use max frames per slice suggested by apple for play audio after lock screen on ios
adjust audio engine can auto-resume from suspend state for ios.
add UnPause and real Pause method on ApplicationMessenger
change to use PRINT_SIGNATURE() for object-c common method trace, add debug log for important methods
add background mode/flag in the main Application and used in ios
accept control from ios control bar (on lock screen and left of multi-task bar)
support show nowPlaying item info on ios lock screen and media control bar
handle ios interruption and app active/inactive/bg switch perperly.

also suspend audio engine when needed, which affect the status of
play-pause button on ios media control bar.
disable ios network auto-suspend after lock and screen goes dark
[IOS] do not keep network alive in idle state (paused or stopped).
[IOS] media controlbar add support seek forward and rewind.
Merge pull request #2130 from ulion/ios_play_background

ios support play background and native media control bar