Fix failing BluetoothOppUtilityTest

In some devices, OPP is disabled, which makes some OPP activities
are disabled accordingly.

Since BluetoothOppUtilityTest doesn't need to launch the actual
activities, this CL makes mockContext.startActivity() do nothing.

Bug: 325674145
Test: atest BluetoothOppUtilityTest
Flag: EXEMPT, test-only change
Change-Id: I546054c54c309cbc4fa35b48ec1740d14d347a82