init: Add a boot monitor for the second stage init

The boot monitor detects early boot lock-up issues before RescueParty starts.

Test:
fastboot oem cmdline set androidboot.boot_timeout=30
fastboot oem cmdline set androidboot.boot_timeout=180

Bug: 383565535
Change-Id: I67d69eac211423c946f0ea62acd0dff525593ea7
Signed-off-by: Jing Wang <jwangh@google.com>