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
2 files changed