Add a flag dot_keep_unusable_xport_sec

Ununsable Transports stay in DnsTlsDispatcher's map for at
least 5 minutes. In order to test that DnsTlsDispatcher can
really remove expired and unusable Transports, make the timeout
configurable via experiment flags. The flag is effective in
userdebug builds only.

This change is a follow up for aosp/2096574.

Bug: 233345976
Test: atest
Change-Id: I670c32e1eda68909296067605f532b2e4c5e8feb