Handled IPv4 without prefix length correctly

LinkAddress always takes address with prefix length. We should
add a prefix length to those IPv4 addresses. This is a regression
from aosp/586706. Also fixed the incorrect regex used to split the
addresses from the modem.

Test: Telephony sanity tests
bug: 72039489
Merged-In: I31fa1a343036370b28ce44fd895802804342dadd
Change-Id: I31fa1a343036370b28ce44fd895802804342dadd
(cherry picked from commit bf307ac1)