libhwui: Fix a native crash with patch caching

This commit makes the system silently deal with requests to remove
patches that have already been tagged for removal instead of dumping.
The garbage collector method (PatchCache::clearGarbage) should deal
with getting rid of this patch at a later date regardless since it
has already been added to the garbage array.

Change-Id: I9bd801926bc5de5cdb0324c9ddeb91bd8bbfe155
Solve a possible NPE in the Documents provider

This commit follows an internal bug report to fix a NPE that would be
thrown in specific situations on specific devices. All callers should
be able to handle the returned null value but if they don't, we can
worry about that later down the line. This null value should not be
returned under normal circumstances anyway.

Change-Id: I274844bf07733643923d5729259f1d9d9cf75e54