Migrate FlickerPiPTests to WmShellFlickerTests.

- This CL migrates FlickerPiPTests.java to WmShell: http://shortn/_Y4GUnepWZl
- This introduces ImeAppHelper and ImeActivity from FlickerTests.
- This uses launchActivity() function to launch Keyboard Activity, since UiAutomator doesn't support to launch the multiple Activities in a task: http://shortn/_KlCIzvahyr

Bug: 167233905
Test: atest WMShellFlickerTests:PipKeyboardTest
Change-Id: I990e6e83213610ca657dbc69f4535971760fde65