Append log data to tombstones

The Android Problem Report site shows tombstones uploaded from
devices.  We can see the native stack traces for every thread,
but sometimes there's a very important bit of information sitting
in the log, and without it we can't analyze the failure.

This change modifies debuggerd so that the log contents for the
crashing process are appended to the tombstone.  The format matches
the output of "logcat -v threadtime".  Both "system" and "main" logs
are included (but not interleaved -- we're not that fancy).

This feature is only enabled when the "ro.debuggable" system property
is set to 1 (indicating a development device).

Bug 5456676

Change-Id: I3be1df59813ccf1058cec496a906f6d31fbc7b04