Use a RwLock for DB_PATH

The return value of DB_PATH.lock() was being borrowed, which holds the
lock for the duration of the borrow.
This is not itself a major problem, but if anything else blocked DB
object initialization, other threads could be blocked for a long time
until initialization completes.

Bug: 184006658
Test: KeyStoreTest
Change-Id: I585b40b8770b90fe80d6591157525eed0b5124c3
2 files changed