Previously all values are cached in the same map, which is inefficient
for Config.getStrings queries which request for multiple settings under
the same prefix and we have look-up the whole cache multiple times. It is
better to have per-prefix caches which only requires one look up per
prefix, and subsequent searches inside the prefix cache would be faster
as well because each per-prefix cache is much smaller.

BUG: 316866951
Test: atest CtsDeviceConfigTestCases CtsPermissionUiTestCases CtsDisplayTestCases CtsAutoFillServiceTestCases SettingsProviderTest CtsProviderTestCases

Change-Id: Ifb4ff91cc034405227d9614bf1029d2a70901bce