Throw XmlParserException when client tag cannot be found when parsing

slice permissions.

There are three call-sites of SliceClientPermissions#createFrom:
1) SlicePermissionManager#writeBackup and
SlicePermissionManager#readRestore ensures the client tag exists before
calling SliceClientPermissions#createFrom.
2) SlicePermissionManager#getClient is expecting Exception from
SliceClientPermissions#createFrom when it fails to parse the xml, and
creates clean permission object accordingly. This CL ensures an
exception was thrown when the xml parsing cannot be completed.

Bug: 121144871
Test: run test suite in forrest
frameworks/base/services/tests/uiservicestests/src/com/android/server/slice

Change-Id: Id7a239f16b95d16ba17bf651d474dec86492ef47