apex: respect filename property for apk-in-apex

android_app_import supports filename: property, which overrides .apk
filename.

Now, apex packaging rule respects it too.
(Previously, apex packaging ignored it)

Bug: 152163837
Test: m  (soong tests amended)
Merged-In: I72e2f1c923f4d01c42a87bf2232a025adca4c918
Change-Id: I72e2f1c923f4d01c42a87bf2232a025adca4c918
(cherry picked from commit 39ee119d)