pom2mk: Fix unlisted dependency types

Android Support Library 26+ does not specify dependency types, so parse
all pom files first, then propagate missing dependency types if we've
got the information.

Bug: 64723465
Test: cd prebuilts/maven_repo/android; pom2mk -use-version 26.0.0-beta2 com/android/support
Change-Id: Ieaff757ff198c9a7b4b006623340b382728c1fd4