Use StubProvider for functional tests.

Add a "UiBot" class for driving the UI from tests.
Add a "DocumentsProviderHelper" class for convenient test doc setup.
Update FilesActivityUiTest to use "TargetContext" which is
    necessary in order to perform ContentProvider
    operations.
Fix a bug where CopyTest relied on implicit order of roots.
Don't include guava in tests...since it breaks functional tests (incompatible class def).
Add test coverage for:
- basic roots list.
- basic files list.
- Live updates to files list.
- basic delete operations.

Bug: 24988170
Change-Id: I2ec01a5e1a474314cb33efb6e92df0f61dfcc1da
NOTE: This is currently broken at the point I try to init files in the stub roots.