Use apex and jar to select boot image dex jar

Previously, only the <jar> part of the boot image configuration pair of
<apex>:<jar> was used to select the module that provides the boot image
dex. The name was sufficient to select the module but not the module
variant and it relied on the platform variant not being installed to
select a unique variant. Unfortunately, when modules are provided as
prebuilts they are not installed and so they get ignored.

This change removes the install check and instead checks the <apex> as
well to select a unique variant.

It also moves the DexJarBuildPath() and ApexModule checks to after
checking the name and treats them as errors. It is safe to do so
because every module that has the same name has to have the same module
type and in order for the module to be valid in the boot image
configuration it must pass those two checks. Treating these as errors
rather than silently ignoring them makes it easier to diagnose problems
with the configuration and/or the boot image modules.

Test: m droid
Bug: 171061220
Change-Id: Ie51f097e081907ea5b75d75840736b10d8d883e8