Treat "GMT" persist.sys.timezone as unset.

Previously, telephony coded depended on
persist.sys.timezone not being set as a signal
to try to deduce a timezone from sim card information.

Due to recent change, timezone is set by default to
"GMT" value. This value is synthetic and never occurs
when set from other sources.

Test: flashall & FrameworksTelephonyTests
Bug: 64056758
Change-Id: I219c2745aff38687b5039ce47eb9f5de3ff57256
(cherry picked from commit 5eb42db3)