Add null check in ScanManager.downgradeScanModeFromMaxDuty

In function ScanManager.downgradeScanModeFromMaxDuty,
ScanClient.stats.setScanDowngrade is called while ScanClient.stats maybe
null. This is resulting in a NullPointerException. All other functions
inside ScanManager using ScanClient.stats perform a null check
beforehand, and therefore a null check is added to fix the NPE issue.
Unit test to check NPE fix is added as well.

Bug: 279848544
Test: atest BluetoothInstrumentationTests
Change-Id: I2dc50d15f7c45b2cc954c00b8ccc4f4313d7e6da