APEX can be flattened

When TARGET_FLATTEN_APEX is set to true, APEXes are flattened, which
means files in an APEX is not packaged into the mini file system image,
but instead directly copied to the system partition.

This option is for devices where kernel does not support loopback
devices or the maximum number of loopback devices is too small (though
the threshold is TBD as of now).

This CL also fixes a bug that jars having bytecode are installed instead
of those having dex.

Bug: 118485880
Test: TARGET_FLATTEN_APEX=true m apex.test; tree
out/target/product/.../system/apex/apex.test shows list of files in it.

Test; m apex.test, then a file out/target/product/.../system/apex/apex
.test.apex exists.

Change-Id: I5a3d62d392d05f2779c4925388afe4f6e460059b