Add `Always_use_prebuilt_sdks` into variableProperties

Unbundled build needs switch mechanism between prebuilt and source.
So add Always_use_prebuilt_sdks because Unbundled_build doesn't gurantee
prebuilts is used.

Bug: 264498267
Bug: 246897249
Test: build
Change-Id: I069d4d9e804b4cf96ae70855c7eee3a6c940c9dc