Fix debugger calling new String().

Used to return an empty string instead of the newly created String.
Requires accompanying jdwp test fix to test String contents.

Bug: 30951794
Test: mm -j24 run-jdwp-tests-host

Merges two cherry-picks:
f15ec058
9e087849

Change-Id: Idc4725b82798a9b6d636c72e76715bc97ad30d23