aconfig: throw exception if reading from DeviceConfig fails

Reading value from DeviceConfig may fail if the provider is not ready.
Since DeciceConfig, and Settings class are static wrapper on top of
the provider, it won't cause the instance  initialization issue.
Thus when the issue happens it means the provider is not initialized.

Test: atest aconfig.test.java and manually make a change in the
framework and check the exception message
Bug: 299471646

Change-Id: Ic08d7a9cd32a8810a7274b6d93c249abccb50b9e