DhcpClient: guard against failure to parse packets

DhcpPacket.decodeFullPacket() is not exception safe and can throw
various runtime exceptions when trying to parse malicious or malformed
packets.

This patch adds a generic catch-all-exception in DhcpClient to avoid
propagating the exception and killing the framework process on reception
of such malformed packets.

Bug: 31850211
Change-Id: I2e723a792ff067ada2834da875700d4df16c5159