Rename nativeLibInfoProperties.archType to archSubDir

Previously, the archType field was a string not an android.ArchType and
was used as the subdirectory into which arch specific files were
copied. This renames the field to archSubDir and clarifies the usage.

Bug: 195754365
Test: m nothing
Change-Id: Id4d6de12b40ee21e41bcd8b96a0ff04eaf615a80