Add unit tests for ProxyManager

- Eliminate redundant parameters in ProxyManager#registerProxy
- Also restrict visibility for some methods by making them private.
Convert  other methods that were package-private to public to be
consistent internally and with other classes like A11yWindowManager

Bug: 262244375
Test: atest ProxyManagerTest, AccessibilityManagerServiceTest,
ProxyAccessibilityServiceConnectionTest

Change-Id: I793d23a11a71d931d270fa4482f44af53e21db47