Harden against invalid paths.

Don't write them, and also, catch runtime exceptions during
persist operations.

Bug: 120749813
Test: atest frameworks/base/services/tests/uiservicestests/src/com/android/server/slice/SlicePermissionManagerTest.java
Change-Id: Ie478814abf5b44e48c8179bee79578e90f4fe3d4