Change default temp allowlist duration for BOOT_COMPLETED broadcast to 20 seconds.

This default temp allowlist duration is used by broadcasts BOOT_COMPLETED, PRE_BOOT_COMPLETED, LOCKED_BOOT_COMPLETED, MY_PACKAGE_REPLACED, TIMEZONE_CHANGED, TIME_CHANGED and LOCALE_CHANGED
that are exempted from BG-FGS-start restriction.

Bug: 187196777
Test: Build and run.
Change-Id: I80aec4cebaff1c6284a3e541e3f4a7a41be2e836