[Android] Add null check to prevent crashes when long clicking a text entry in ListVi...

[Android] Add null check to prevent crashes when long clicking a text entry in ListView header/footer (#271)

When a text entry control (Entry, Editor, SearchBar, etc.) was being used in the header or
footer of a ListView on Android, a long click/press would cause a crash. This was occurring
in the HandleContextMode method because it expected to be a cell. Adding a null check and
breaking out of the method if the value from GetCellForPosition is null prevents this crash
from occurring.