Add AccessibilityNodeInfo.isTextSelectable

ACTION_SET_SELECTION is needed on all TextView nodes to reset the
cursor. But some non-edit texts are selectable and need to be
identified by a11y services. ACTION_SET_SELECTION has been on these
nodes for years so to avoid the complexity of breaking this, add a
node property.

Services should use ACTION_SET_SELECTION for nodes where
property=true

Bug: 62058901
Test: atest  AccessibilityTextTraversalTest, TextViewTest cts and
core (TextViewTest#testCutShouldNotThrowException keeps failing but
it's unrelated to this change)

Change-Id: Ia825dd806824e663e7fa7d274f51c6bf44f0c2c4