PersistableBundle.restoreFromXml should return a mutable bundle.

Issue:
  Currently, restoreFromXml/readFromStream returns an immutable
  PersistableBundle.EMPTY when the underlying XML file is
  ill-formatted. Otherwise it returns a mutable one.
  This inconsistent behavior easily causes crash when returned
  PersistableBundle is mutated.

Fix:
  Make restoreFromXml consistently return a mutable
  PersistableBundle.

Bug: 185436202
Test: make
Change-Id: I3c9ab4df9f0fea2d46d1c9b72ffd4b65790180b2