Make sure last top time is valid.

In some cases, the app can start an FGS due to certain launch
exemptions without the app being in the TOP state. If this is the
case, the app's last TOP time will always be Long.MIN_VALUE, causing
the handler to repeatedly send messages and leading to a deadlock.

Fixes: 337740958
Fixes: 337701437
Test: atest CtsFgsTimeoutTestCases
Change-Id: Idf303b7acfd73f46481309029816686a5161ada7