Improve logging for why a staged session was reverted

mFailureReason gets populated with actual reason when we try to resume
the staged session in checkpointing mode. But, if the device has a
native crash due to which the device reboots even before we attempt to
"resume" staged session, then it will never get populated.

As such, during boot loops that occur earlier than "resume" of staged
session, we end up logging nothing.

In this CL, we improve on the scenario by falling back to using
mNativeFailureReason if mFailureReason is empty. But this only works if
our staged session contains APEX package in it. For apk-only staged
sessions, we will still be logging blanks.

Bug: 163787002
Test: atest ApexRollbackTests#testReasonForRevertIsLoggedDuringBootloop
Change-Id: I5cd1733d66aa0b30855803b95eb9818dc81aa0d7