VTS: powerstats: skip GPS power stats

Lassen GNSS power stats will be present after running GPS session
once. Otherwise, VTS will fail due to missing GPS power stats.

Bug: 325545917
Test: run vts -m VtsHalPowerStatsTargetTest
Change-Id: I9d31017a86417fc18f4eab98ee25abfadfa396f9
Signed-off-by: Darren Hsu <darrenhsu@google.com>