SRTP: Lower SDES key lifetime minimum to 2^20

SRTP SDES key lifetime support was added in ASTERISK_17899.

In that addition, the minimum key lifetime to be accepted was
set at the 10 hours @ 20ms/packet = 1800000 packets.

The firmware in the obi1xx ATA uses a hardcoded lifetime of
2^20 packets.

Lower the limit to 2^20 to support a wider field of clients.

ASTERISK-27967 #close

Change-Id: I81a0703c595a0c9101dfdf02300149a3cc39bf94