Refactor LE scanning HAL (1/3)

This patch converts btgatt_scanner_interface_t struct into
BleScannerInterface class. It also refactors three most important
methods from this interface: RegisterAdvertiser, Scan, and Unregister.
Rest of this interface will be updated in following patches.

Bug: 30622771
Test: sl4a BleScanApiTest
Change-Id: I0b7dddc1e9906d825096e23a83a39ecc681d232c
diff --git a/include/hardware/bt_gatt.h b/include/hardware/bt_gatt.h
index 414d3e4..393d1a5 100644
--- a/include/hardware/bt_gatt.h
+++ b/include/hardware/bt_gatt.h
@@ -61,7 +61,7 @@
     const btgatt_server_interface_t* server;
 
     /** Pointer to the LE scanner interface methods.*/
-    const btgatt_scanner_interface_t* scanner;
+    BleScannerInterface* scanner;
 
     /** Pointer to the advertiser interface methods.*/
     BleAdvertiserInterface* advertiser;