Separate logic for alg selection for OTA and APK signing.

The rules for which digest algorithms are accepted by the Android
platform for APK signatures and OTA update package signatures are
different. For example, the set of digest algorithms accepted for APK
signatures depends on the signing key algorithm and the platform's
API Level. Whereas the set of digest algorithms accepted for OTA
update package signatures by Recovery depends on the list produced
by the build system, which in turn produces the list based on the
key algorithm and digest used in the signing certificate.

To reflect this reality, this refactoring CL explicitly separates
signapk's logic for choosing the digest algorithm to use for v1
signing from its logic for choosing the digest algorithm to use for
OTA update package signing.

Bug: 28296599
Change-Id: Ic7aa77e89622d727e985f8749071284746be7f45