Intern the strings in ApplicationExitInfo

This reduces the memory usage of AppExitInfoTracker by nearly half.

Also reformat the dump output of it.

Bug: 204471732
Test: atest ApplicationExitInfoTest
Test: atest CtsAppExitTestCases:ActivityManagerAppExitInfoTest
Test: Manual - dumpsys activity exit-info
Change-Id: I659060d83e2e1072f8aa885a41256ca7ff86808a