Remove magic number 0x0102000b from MockIme

This is a follow-up CL to our previous CL [1], which enabled MockIme
to be used to test inline-autofill integration with
InputMethodService.

In order to enable InlineUiBot to look up LinearLayout that hosts
InlineContentView, the previous CL assigned a view-ID 0x0102000b,
which corresponds to "android:id/message".

This CL re-implements that logic by letting InlineUiBot to look up a
View with its content-description instead of its view-ID.  In this
way, we can more easily separate concerns between how MockIme can be
configured and why certain MockIme configurations are chosen.

There should be no change in the actual test expectations.

 [1]: I7b5772550127f840811e7f39db38a404f9d4c29a
      a99b496f2268fe0e1f55220f03524a3f0f343b8f

Bug: 149926050
Test: atest CtsAutoFillServiceTestCases
Test: atest CtsInputMethodTestCases
Change-Id: If8ed65951f942e10364550fcea92ec6f8ec558a6