Workaround for preserving responsiveness on a slow InputConnection.

1. Add mechanism to detect a slow or non-resonsive InputConnection (IC)
2. When IC slowness is detected, skip certain IC calls that are known
   to be expensive (e.g., getTextAfterCursor).
3. Similarly, disables learning / unlearning on a slow IC.
4. IC slowness flag is reset when starting input on a new TextView or
   when a fixed amount of time has passed.

Note: These are mostly temporary workarounds. The permanent solution is
to refactor RichInputConnection so that it is less sensitive to IC
slowness in general.

Bug: 21926256
Change-Id: I383fab0516d3f3a8e0f71e5d760a8336a7730f7c