Make implementation of isEmpty consistent with implementation of getCount in HeaderListViewAdapter

Fix for Issue ID 18316. Make isEmpty() return false when getCount() returns a non-zero value. So that the list cannot be considered empty when a header or footer is present.

https://code.google.com/p/android/issues/detail?id=18316

Change-Id: Ib69201e9d5ef3efcbb68ea298b8cc8ca6e027246
Signed-off-by: Emma Sajic <esajic@effectivelateralsolutions.co.uk>