Remove abstract from AccessibilityService methods

Two methods on AccessibilityService were declared abstract, but
there doesn't seem to be a good reason for them to be abstract, as
it's possible to write an accessibility service that doesn't do
anything useful with the methods. Removing the abstract qualifier
to reduce boilerplate for service developers.

Bug: 31705365
Test: Trivial change, so I just verified that it builds.
Change-Id: I2af39848c8ab343d562361fdc1c3f8ca3ecb5f00