Mark TelephonyNetworkFactoryTest flaky

Test is currently failing, presubmit sees this and ignores other test
failures because it considers the telephony test suite to be flaky (e.g.
aosp/1615646 and ag/13892068.

The test failure is not reproducible for me locally, so I'm fixing by
marking @Flaky to exclude this test from presubmit.

Test: atest
Change-Id: I98520b25ce094bf97f38a2606195d38086083220