Arch-specific source for prebuilt_apex

Arch-specific source can be specified for prebuilt_apex as follows.

arch: {
    arm64: {
        src: "myapex-arm64.apex",
    },
},

A note on the implementation. The Src property was not tagged as
`android:"arch_variant"` as usual. Instead, multiple Src properties are
explicitly declared like

struct Arch {
    struct Arm {
        struct Src *string
    }
    ...
}

Corresponding Src property is manually selected according to the
MultiTargets()[0].

This is because prebuilt_apex is mutated only for android_common, in
order to have the same arch variant with the apex module type.
Therefore, we can't rely on the arch_variant tag.

Bug: 127789981
Test: m (apex_test amended)

Change-Id: I77dbe626171d8975f549bdb4af3c487232cf05f7