Calculate the runtime fingerprint prefixes from build prop

In practice, some partners use the 'import' statement to override
the device fingerprint at runtime. The runtime fingerprint will
later add to the metadata of OTA package, so that the OTA server
can deliver the package to corresponding devices correctly.

This CL supports parsing a subset of import statement that the init
process recognizes. And we loose the restriction based on how the
dynamic fingerprint is used in practice. Right now, we only searches
for the override of brand, name and device. And the placeholder
format should be ${placeholder}, with its value supplied by the
script caller.

As part of the implementation, we generate all the possible
combinations of the input boot variables. And recalculate the
fingerprint for each of the combination. Though we load the
build.prop multiple times, the logic is easier to follow. Also,
it's more convenient to enhance the logic if we only want to
allow some of the boot variables combination later.

Bug: 152167826
Test: unittests pass
Change-Id: Iac093a40dc6f873c5e1858efa44cb2bd6082508a