[Nearby] Do not invalidate BlePeripherals during discovery.

BlePeripheral references must remain valid even after the peripheral is
no longer found during a scan.

Bug: 1135801
Change-Id: If3df9f9de414760fc6a00bebfff94a8861fdeb6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2454378
Auto-Submit: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: James Vecore <vecore@google.com>
Reviewed-by: James Vecore <vecore@google.com>
Cr-Commit-Position: refs/heads/master@{#814486}

chrome/services/sharing/nearby/platform_v2/ble_medium.cc

@@ -319,8 +319,6 @@ void BleMedium::DeviceRemoved(bluetooth::mojom::DeviceInfoPtr device) {
 
     it->second.peripheral_lost_cb(*ble_peripheral, id_uuid_pair.first);
   }
-
-  discovered_ble_peripherals_map_.erase(address);
 }
 
 void BleMedium::AdvertisementReleased(