[SettingsProvider] add generation registry stats to dumpsys

Include some basic stats such as cache size on the server side.

BUG: 228619157
Test: adb shell dumpsys settings
Test: adb shell dumpsys settings --proto | protoc --decode android.providers.settings.SettingsServiceDumpProto frameworks/base/core/proto/android/providers/settings.proto

Change-Id: Ie621167932aa4b4d9fd8acc5999784e1f2ddb027