Prevent exception when surrounding text retrieval

We use same reference from TextView to set the initial
surrounding text. The actual surrounding text may be
modified before retrieval since the mSurroundingText
is mutable. Use a copy of subText should avoid this
concurrent issue.

Bug: 160390184
Test: atest FrameworksCoreTests:EditorInfoTest
Change-Id: I6082a4cae2fcdc4c529dc14e2e5e7a45ab1aae4d
(cherry picked from commit 0ebe70cb)