[SettingsProvider] only increment generation of unset settings if new setting is non-predefined

This is a small optimization for the previous CL. The generation number
of all non-predefined, unset settings should only be incremented if the
newly inserted setting is also non-predefined. If it's a predefined
setting, it doesn't affect the non-predefined, unset settings.

BUG: 228619157
Test: atest android.widget.TextViewPrecomputedTextPerfTest#testOnMeasure_RandomText
Test: observed no regression
Change-Id: I33e2391e3e07fb5251cb497a0e87e9815d540e55