Use separate receivers for managing the intent registerations

Add the `IntentReceiver` class, and modify the `PairingTest`
- The intent registration and verification in `PairingTest` is now done through the newly added `IntentReceiver` class.
- Added documentation for the class, and the usage in the `PairingTest`.
- Modified the `PairingTest` accordingly to use the `IntentReceiver` class, which makes it cleaner and more concise.
- All the existing test cases pass as is, with no changes.

Test: atest PairingTest
Flag: TEST_ONLY
Bug: 388810455
Change-Id: I2f440bbce2fc83cf6cc3cbedf9061e8eb80e8a42