API and boot image updates to import RI BigInteger

This reorders methods, omits a redundant "implements" spec, and adds
<x>ValueExact() methods, which are in OpenJDK 8, but hadn't been
implemented on Android.

Also update the boot image profile to compile BigInteger-required
methods that used to be native. Some of these must be compiled to
avoid ART test timeouts.

Bug: 136887041
Bug: 119491938
Bug: 28214673
Bug: 28251030
Bug: 2950143
Test: AOSP Boots. Ran some manual Calculator tests on Cuttlefish.
Change-Id: Idd72ec6a6fd54ed4ad96a82b26840b41a3882678