Refactor in PositionTesterContextMenuListener.

The asserts were removed from that class and replaced by a status String object.

This allows ExpandableListTester to do the asserts instead.
These tests passed on a sapphire and passion devices as well as in the emulator.
The asserts in the main thread are expected to make these tests pass during the
continuous build too.

Change-Id: Ic6d23e1d3a151f25249eed553113666f47b40dc3
http://b/issue?id=2525846