Remove idmap path 256 length limit

Overlay and target package paths can be longer than 256 characters.
Currently, the idmap will fail to be generated if either path
is longer than 256 characters.

This change removes the 256 character limit and makes parsing variable
length strings easier in libandroidfw.

Bug: 174676094
Test: idmap2_tests && libandroidfw_tests
Change-Id: Ic240cdb8700566b2ac2ade08da58bea852e4ae0c