Add Android-changed markers to Throwable.

With one exception, this is just adding appropriate comments around
the existing changes vs upstream (version 8u121-b13). There are some
changes which look like they could be undone, but previous attempts to
do so caused issues with serialization of some subclasses, and those
attempts had to be reverted (see http://r.android.com/703384 and
http://r.android.com/703085).

The exception is in the implementation of getOurStackTrace(), where
the changes to the upstream code are refactored slightly to make the
delta easier to see.

Bug: 35910877
Test: `cts-tradefed run cts-dev -m CtsLibcoreTestCases`
Test: Manually check for serialization exceptions on startup via logcat
Change-Id: Ia4b25dae08b8c704457dc32687a25420ae80590d
1 file changed