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
diff --git a/libs/androidfw/Idmap.cpp b/libs/androidfw/Idmap.cpp
index 68844be..73e040c 100644
--- a/libs/androidfw/Idmap.cpp
+++ b/libs/androidfw/Idmap.cpp
@@ -288,7 +288,8 @@
   if (!target_path) {
     return {};
   }
-  if (!ReadString(&data_ptr, &data_size, "debug info")) {
+  if (!ReadString(&data_ptr, &data_size, "target name") ||
+      !ReadString(&data_ptr, &data_size, "debug info")) {
     return {};
   }