MediaRouter: don't scanPackages if unnecessary

scanPackages was called whenever it receives a broadcast,
which could be duplicated.
This CL adds a boolean variable (mScanPackagesScheduled) not
to scan packages if it is scheduled.

Bug: 153527463
Test: cts test, atest mediarouertest and check
MediaRouteProvider#onBind, #onUnbind are called
when a new provider is installed.
Confirm # of calls is reduced (3 -> 1).

Change-Id: Iaa423b7d1b2b2a97aebed0001b4077bdf95b1d54