ART: Remove incorrect HFakeString optimization

Simplification of HFakeString assumes that it cannot be used until
String.<init> is called which is not true and causes different
behaviour between the compiler and the interpreter. This patch
removes the optimization together with the HFakeString instruction.

Instead, HNewInstance is generated and an empty String allocated
until it is replaced with the result of the StringFactory call. This
is consistent with the behaviour of the interpreter but is too
conservative. A follow-up CL will attempt to optimize out the initial
allocation when possible.

Bug: 26457745
Bug: 26486014

Change-Id: I7139e37ed00a880715bfc234896a930fde670c44