Created new Settings preferences for internal autofill state.

Currently, these preferences can only be changed through 'cmd autofill', but
moving them to the Settings provider will let us manage them through the
Settings -> Developer Options screen:

- AUTOFILL_LOGGING_LEVEL
- AUTOFILL_MAX_PARTITIONS_SIZE
- AUTOFILL_MAX_VISIBLE_DATASETS

Test: manual verification
Test: atest CtsAutoFillServiceTestCases FrameworksCoreTests:SettingsBackupTest

Bug: 65700540

Change-Id: I6c864d032bc0b2c3b7c24f300af37883adbaf0d0