This fixes the bug: flattened apex is installed under
/system/apex/<soong module name> not /system/apex/<apex name>.
Flattened apexes can be seen as pre-activated under /system/apex, which
is bind-mount to /apex. Therefore, install dir for flattend apex should
be the same with the activation point(or mountpoint) of non-flattened apex.
When apex_name: is set, it should be used as its name instead of name:.
This change also emits makevar APEX_FILE_CONTEXTS_INFOS which is list of
pairs(<apex_name>:<file_contexts>) and is used by sepolicy to modify
file_contexts files for flattened apexes properly.
Bug: 123314817
Bug: 142300241
Test: add a test apex module(foo) with apex_name(foobar)
Test: OVERRIDE_TARGET_FLATTEN_APEX=true m foo
Test: see if the apex is installed correctly(/system/apex/foobar)
Test: see if the file_contexts for foo is correctly modified
Change-Id: I97a07de5cc772cd1d91e95ae059d282038028916
7a78a92f79