Remove unnecessary Base64 encode and decode

The HAL has returned base64 for SIM authentication. Therefoere,
it's unnecessary to decode in RadioResponse and re-encode it at
the end of getIccSimChallengeResponse.

Bug: 149492696
Test: CarrierApiTest, SimRestrictedApisTest, IccRecordsTest
Change-Id: I9ef5578b434bdf61606dc8075363fc31988b611f
Merged-In: I9ef5578b434bdf61606dc8075363fc31988b611f
(cherry picked from commit 01016f7b)