Set min_sdk_version for java imports in pom2bp

Previously min_sdk_version was only set for *_library build rules, and
not for *_import targets. This made min_sdk_version default to
sdk_version, which is usually "current", which is not correct for these
libraries.

Test: pom2bp -default-min_sdk_version 24 for androidx
Change-Id: I3b029315abeb559e66efad847ae664efb79f4509