Parse fingerprint and timestamp unconditionally

build_image.py adds `fingerprint` and `timestamp` to the config object
by doing some string processing, but does so only when mount_point is
missing in the input property file. This makes this unavailable to
Soong's android_filesystem modules, which always provide a mount_point
in the property file.

This CL creates a `TryParseFingerprintAndTimestamp` function to make
this logic common to empty and non-empty `mount_point` property files.

Bug: 376539388
Test: lunch aosp_panther-trunk_staging-userdebug
Test: m
out/soong/.intermediates/packages/modules/Virtualization/guest/pvmfw/pvmfw_img/android_arm64_armv8-2a_cortex-a55/pvmfw.img
(a bootimage module) followed by rm repeatedly
Test: sha remains consistent

Change-Id: I602815338d8938cc81ba44d2e9aa1a9ff8afccde