Additional CTS test for pinned stack

- Verify that pinned stack is front stack.
- Verify that there is only one resumed activity in the system.
- Verify that pinned activity window is the front window.
- Verify that pinned stack isn't focused stack and is for always focuable activity.
- Verify that pinned activity isn't focused activity and is for always focuable activity.
- Verify that pinned activity isn't resumed activity and is for always focuable activity.
- Verify launch on pinned stack @hidden API functionality.

Also, made the host side test class more modular.

Bug: 19225708
Bug: 26273032
Change-Id: Iab88787cba786737598eb7ea4137e067c6ec5f48
11 files changed