Add FeatureOverrides::toString()

Add FeatureOverrides::toString() to output a string representation of
the current FeatureOverrides values. This is required when implementing
Parcelable (in a later patch) as well as for 'cmd gpu featureOverrides'
(also coming in a later CL).

Bug: 372694741
Test: CQ
Test: Manual verification with later CLs
Flag: com.android.graphics.graphicsenv.flags.feature_overrides
Change-Id: I77ca945f173ae1ad5430670c87e46985189409cb