Use Uppercase instead of Lowercase for PLMN Decode

A change to IccUtils converted from using lowercase
to uppercase and broke the PLMN trimming logic.

This resolves bugs where the platform may report
5-digit PLMNs with an invalid trailing 'F' character.

This fixes an issue introduced by aosp/575243, which
impacts the Manual Network Selection menu and
the public API.

Bug: 79561854
Test: compilation
Merged-In: I5ea7867cd9c11fe4454188fd1f30bf58b2911712
Change-Id: I5ea7867cd9c11fe4454188fd1f30bf58b2911712
(cherry picked from commit b70fbc85)