Call fscrypt_destroy_volume_keys() under mCryptLock

Everything in FsCrypt.cpp seems to run under VolumeManager::mCryptLock,
except for fscrypt_destroy_volume_keys() which uses mLock instead.

This was sort of okay because fscrypt_destroy_volume_keys() didn't
operate on any in-memory data structures.  However, that is going to be
changed.  Therefore, rework VoldNativeService::forgetPartition() to call
fscrypt_destroy_volume_keys() under mCryptLock.

Test: see I7f11a135d8550618cd96013f834cebd54be5ef84
Change-Id: Ia27a61faf2fdd546cdbddb2a3985c7c6696f6aa6
Merged-In: Ia27a61faf2fdd546cdbddb2a3985c7c6696f6aa6
(cherry picked from commit ce86e24d)