Accept --overlay-name flag in idmap2

To support allowing for multiple <overlay> tags in one package, idmap2
must be able to generate an idmap for an individual <overlay> tag.

`idmap2 create` now accepts a --overlay-name flag that specifies which
tag to use to generate the idmap. The value of --overlay-name should be
set to the value of the android:name attribute on the <overlay> tag to
use.

If the flag is not present, idmap2 will look for an <overlay> tag with
no value for android:name.

Bug: 162841629
Test: libandroidfw_tests
Test: libidmap2_tests
Change-Id: I02316d0b88773f02c04a5d462be9825016fa496d