storageproxyd: Start only a single binder thread

We only need a single binder thread to receive notifications, so we
should set the thread pool max to 0 rather than 1. startThreadPool()
starts one thread + the max count.

Test: m storageproxyd
Bug: 281951047
Fixes: 292030372
(cherry picked from https://android-review.googlesource.com/q/commit:b05b8706714a6e7eb64890e0bee012dbc69f72be)
Merged-In: I53a90eaa2aa69469fd3a00b6da0d7061318c8ba9

Change-Id: I53a90eaa2aa69469fd3a00b6da0d7061318c8ba9