findNetworkStackApk returns an array of files (2/n)

This change handles the case of split APKs. All files starting with
"NetworkStack" will be returned as split APKs.

See b/159978937#comment6. NetworkStack is changed from a single APK to
split APKs. We need this change for install to succeed.

Bug: 159978937
Test: atest NetworkStagedRollbackTest
Change-Id: I20f5fee08ea208ba4dc7711d82a1718239c647c2