Add flicker test for WM Shell

Separated folders in libs/WindowManager/Shell/tests
- unittest, for unit tests
- flicker, for this flicker tests
- propose to add functional/ folder for functional tests

Note also
- Copied over EnterPipTest from existing flicker tests as proof of
concept
- TODO: need to figure out the next step for the flicker test library
itself as it's currently packaged in com.android.server.wm

Bug: 161561007
Test: atest WMShellFlickerTests
Test: atest WMShellUnitTests
Change-Id: Ic06e7cb0f98151591ccb9f5217260939349a9aa5