Dynamically show the menu button on the system bar.

Windows with FLAG_NEEDS_MENU_KEY (or windowNeedsMenuKey=true
in their theme) will cause the system bar to show a menu
icon. (Note that the phone's status bar currently ignores
this, but phones tend to have hardware menu keys anyway.)

Additionally, all windows whose package's SDK version is
pre-Honeycomb will have FLAG_NEEDS_MENU_KEY set by default.

Bug: 3003728

Change-Id: I2d983763a726ea4f32cd1af9b0390e30478b11d1