Refactor: AddDataApex

Extracted AddDataApexFiles() from AddDataApex(). AddDataApexFiles() will
be used by the upcoming change to add block-device apexes backed by
dm-linear devices.

Bug: 385834023
Test: ApexTestCases
Change-Id: I88ef0785decb52aaed4e273dfe78ca8fcbf67fb4
2 files changed