Defer the Pss sampling on app processes post the boot

Now we're not going to collect the Pss for the first N(default=20) mins
after the boot. Given the system is most likely very busy, sampling
the Pss makes the system load even worse.

Bug: 304484667
Test: Manual - check logcat.
Change-Id: I2f7fb78e003fa3b426643f2a7067afb77b5b9b63