Compile stubs to .dex

Although stubs are not installable on device, we need to compile .dex to
support hiddenapi metadata generation.

Bug: 271443071
Test: Built out/soong/hiddenapi/hiddenapi-stub-flags.txt with
aosp/2487266
(There is still some delta due to missing signatures in child classes)
(Will address them in a followup CL)
Change-Id: Iac1330c449934085a479c487a1489aa1695a75ee

Change-Id: I1baa9c0f45eb8fc93f5c3177389ea5a2b7ca0a0b