Adds null checks on callers of Nullable method getA11yServiceInfo().

Also renames mPackageName to mFeatureName. This string is used as the
user-visible feature name, e.g. TalkBack, not the APK package name.

Fix: 382054546
Test: n/a (existing presubmit)
Flag: EXEMPT minor bugfix
Change-Id: I1ce6902612d89bbb68bddf30227ca231ea98dce7