Throw exceptions instead of returning null

This commit changes

1. load methods will throw FileSystemNotFoundException, if new storeage
   is not available on the device
1. load(container, package, storageFileProvider) will throw exception if
   container is null

Test: atest aconfig_storage_package aconfig_storage_file.test.java
Bug: 367765164
Flag: EXEMPT refactor
Change-Id: I81de851793f56691fb41caad03cba491fe7527c0