SystemUI: Add task manager

Add a button in status bar expanded header to switch to task manager,
which supports to watch current running tasks and usage of memory, also
supports to close one or all of running tasks.

Conflicts:
        packages/SystemUI/src/com/android/systemui/statusbar/phone/PhoneStatusBar.java

Change-Id: Ica51961bfac78580eac085c18eb42f91a84efc95