Include the Subreason, if any, in ApplicationExitInfo.getDescription().

Instead of making the Subreasons public, which could introduce
API churn, let's add the string representation of the subreason into
getDescription so that an app developer can relatively easily see those
details if they're interested.

Bug: 266248465
Test: atest ApplicationExitInfoTest
Change-Id: Id4fd8d846886b05302ecfa8fe7f764364ca94f60