Remove boot image begin/size from image header.

Use a single boot image range for both the heap (*.art) and
code (*.oat) boot files.

This is done in preparation for boot image extensions. We do
not want to encode multiple heap and code ranges and shall
keep a single boot image range for the boot image including
extensions.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 119868597
Change-Id: I8baec8361602c445652e69bdfbb57e7a7c7c6419