carrier config: add imsi_encoding_method_int flag

If a carrier supports RFC 4648 encoding method for encoding encrypted
IMSI and SSID, it needs to be set.
By default, we use RFC 2045 encoding method.

Bug: 109795427
Test: manual
Change-Id: I2ab160593a56b8e5fc10857686f0592be94959d3
Signed-off-by: Ecco Park <eccopark@google.com>