Trim Invalid Pad Characters from EF_IMSI

Remove trailing f's from IMSI records and
ensure that if IMSI is unset that the record
is null by convention.

Bug: 115275742
Test: atest TelephonyManagerTest#testGetForbiddenPlmns
Change-Id: Ia71aaf3e183fc8835e1d62e84ea3f0e7df139986