Initial prototype of tiny taskbar.

Introducing isTinyTaskbar and using in a few places to show Taskbar on
phones.

Disabling for three button mode since the buttons will take up most of
the space. Could also prototype adding meta key as a 4th button in
this case if we feel like it.

Flag: com.android.wm.shell.enable_tiny_taskbar
Test: Manual
Bug: 341784466
Change-Id: Iba07382d9bd5d14fc29ef080a390db561f6fd454