gpuservice: Always reset feature overrides when parsing

Always call resetFeatureOverrides() at the start of
parseFeatureOverrides() to remove stale values before adding the newly
parsed values.

Bug: 372694741
Test: atest gpuservice_unittest:FeatureOverrideParserTest
Test: CQ, Manual verification
Flag: com.android.graphics.graphicsenv.flags.feature_overrides
Change-Id: Ie365dd2ffd648089a808f034fde5cc6886570cb8