TopicAutocomplete: Use simple string match for topic filtering
TopicAutocomplete: Use simple string match for topic filtering

Using a RegExp on user input crashes the app on invalid inputs like entering a
single `(`. The search seems to have been using a RegExp search only for
enabling case-insensitive search. This has now been switched to a simple
sub-string test using lower-cased strings.

Fixes #2857

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile