[1/3] Incoming call in background

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This feature prevents the incoming call from backgrounding
the current activity. Which is different from the
"non-intrusive incall ui" feature.
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Add an user configurable option to keep the incoming call
UI in the background when screen is on.
Instead, a notification will be added to notification bar,
which can be used to bring up the incoming call UI.
Ringtone and vibration still occur as per system settings.

Also, the notification added to status bar contains
decline and answer call action. This allows user to
take action directly within the notification without
having to enter the incoming call UI. As well useful
for declining or answering a second incoming call.

[1/3] 后台来电

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此特性可阻止来电强行插入前台而将前台正在进行的活动挂入后台.
与"非侵入式来电UI"并不一样.
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添加一个用户可配置的选项从而在亮屏时将来电UI置于后台.
同时, 会有一条通知被添加至通知栏, 可点击调出来电UI.
铃声及震动(根据系统设置)不受影响.

另外, 添加至通知栏的通知包含接听和挂断按钮. 允许用户直接
在通知栏里进行操作而不用调出来电UI. 同时对于通话过程
中处理新的来电也很有用.

[2/3] in platform_packages_services_Telephony
[3/3] in platform_frameworks_base