Fail gracefully if we get a bad API whitelist.

If we send a bad API whitelist to the Zygote, it causes it to close the
socket. If we take no further action in AMS, it results in the same list
of exceptions being sent when we re-open the socket, resulting in it again
being closed. This results in no longer fork/start any new processes.
Since the list is persisted, this would result in the device entering a
boot loop upon reboot. Since no apps could be started, we cannot recover.

So in the case that the exemptions list causes problems, clear out the
list so we don't try to send it again next time. This means we will see
a single failure, but future attempts will succeed (obviously without
any whitelist). The device should not enter a boot loop.

Note, the test below relies on the fact that we can send at most 1024
arguments in a command to the Zygote (MAX_ZYGOTE_ARGC), and that each
item on the list is a separate argument.

Test: adb shell settings put global hidden_api_blacklist_exemptions \
Test:    $(for i in {1..1025}; do echo -n $i,; done)
Bug: 64382372
Change-Id: Ie47095d516c247ff6a8d667a2ac9b7be45f1acda