Add an output file type for flattened APEXes.

Allow modules to depend on the flattened flavor of an APEX package
using `module.{flattened}` (returning the output install path of the
flattened APEX).

Also add this flattened output install path as an implicit output
dependency of a flattened APEX phony module.

Test: m checkbuild
Bug: 139277987
Change-Id: I8c55d5ff2922a36fcc8ec0819fbe3e93a772ef8a
Merged-In: I8c55d5ff2922a36fcc8ec0819fbe3e93a772ef8a