storageproxyd: Start binder thread pool

The Trusty storage proxy requires that the suspend service is started to
acquire a wakelock for UFS RPMB operations. Without the binder thread
pool running, starting this service results in at least a 1s polling
delay. This change ensures that we start the thread pool before handling
any RPMB operations, so acquiring the wakelock will complete as soon as
the service is ready without needing to poll once per second.

Test: m storageproxyd
Test: Artificially delay suspend_service to check if we poll
Bug: 281951047
(cherry picked from https://android-review.googlesource.com/q/commit:fb92cd3c22980e6ee6574be193f8ec993d19b2bf)
Merged-In: I1a4cdd48d57201b0cf9c24523d22e5bdbcea376a
Change-Id: I1a4cdd48d57201b0cf9c24523d22e5bdbcea376a