Support multiple sources for prebuilt_etc

Keep the Src attribute for compatibility. The new attribute (Srcs) is
mutually exclusive with Src.

Keep SourceFilePath and OutputFile for compatibility with other modules.
These can be removed in a follow up change.

Bug: 328313691
Test: presubmit
Test: m blueprint_tests
Test: prebuilts/build-tools/build-prebuilts.sh (on build-tools branch)
Change-Id: I5d5b2657715a7180a829c7ed0f501872d561b662