Fix long characteristic write concurrency bug.

Tag: #stability
Test: Bramble (pixel phone) with test app
Bug: 169559728
Ignore-AOSP-First: Current security annotation parity
BYPASS_LONG_LINES_REASON: Bluetooth likes 120 lines

Change-Id: I3e3a5ff0965c1ca4f79e70e6163bc5d6917edbd1