Improve module filtering in hiddenapi stubFlagsRule()

Previously, it ignored any module that was not a platform apex variant
however that required every module that was referenced from the boot
jars to have a platform variant which is not the case when building
from prebuilts.

This change switches it to explicitly check that the variant is for
either the appropriate apex or the platform depending on what is
configured in the BootJars or UpdatableBootJars.

It partially duplicates some logic from the getBootImageJar() function.
It intentionally does not refactor the getBootImageJar() to allow for
reuse because coupling the hiddenapi and dexpreopt logic would make
refactoring either of them more difficult. Any duplicated code will be
deduped once they have both been refactored.

Bug: 178361284
Test: m droid
Change-Id: I4b4e0dc8ee40c1ba1713d7ef72df13d175e84af6