[SettingsProvider] tracking generation of non-predefined unset settings

Non-predefined, unset settings will share one generation number, which
increments whenever a new non-predefined setting is inserted. This
allows queries on non-predefined, unset settings to be cached.

BUG: 228619157
BUG: 271355793
FIXES: 271355793
Test: atest NameValueCacheTest
Test: atest GenerationRegistryTest
Test: atest android.widget.TextViewPrecomputedTextPerfTest#testOnMeasure_RandomText
Change-Id: Ia8167321836ab5b6846f11e044611831e80e2704