pom2bp: clean up templates

Move some of the more complicated conditional logic out to helper
functions.  Use the {{- }} syntax to strip previous whitespace to
allow spacing out the remaning conditionals in the templates.

Test: (cd prebuilts/sdk/current/androidx && pom2bp -regen Android.bp)
Change-Id: I766bd0e1837aa04375f322fbe796d923cd99ecde