Close heap dump ParcelFileDescriptor.

The file descriptor is only closed in certain situations. Explicitly
closing it to ensure we don't have any leaks.

Bug: 154277871
Test: take a system heap dump via developer options & and watch for StrictMode violation
Test: adb shell am dumpheap <system|another app> & and watch for StrictMode violation
Change-Id: I3ae3e270bcec18ba531055becfff32924c8f2a2d