Skip not existing VNDK prebuilts

Due to a bug of some past versions of VNDK, invalid VNDK modules without
any prebuilt sources may exist. Skipping them instead of creating
useless modules.

Bug: 154777533
Test: capture snapshot, delete some .so files, and then install Android.bp
Change-Id: I5579c1ae86453d1b6c8738455ff71bf106afbda4
1 file changed