Fix possible deadlock in media provider

Because of the execution sequences below,
receiving ACTION_MEDIA_EJECT intent during bulk inserting
can lead to media provider deadlock.

bulkInsert()
-> db.beginTransaction()
-> insertInternal()
-> getDatabaseForUri()
-> synchronized(mDatabases)

mUnmountReceiver.onReceive()
-> synchronized(mDatabases)
-> db.update()

beginTransaction() blocks any other operations on the database
until endTransaction() is called.
Therefore, db.update() method can be blocked in the synchronized block
and bulk inserting also can be blocked simultaneously to enter
the synchronized block.

This patch resolves the problem above by restricting the scope
of the synchronized block in mUnmountReceiver.

Change-Id: Ibb65ac82ea389145a6a3e44942928b7548c53b22
Signed-off-by: Sangkyu Lee <sk82.lee@lge.com>