Add a showInInputMethodPicker attr.

An app that provided an IME and is deprecating that functionality may
want to still keep it's IME around temporarily, to prevent breaking
clients that hardcode the component name when switching to the IME.
Such an IME may want to not be presented as an option in the Input
Method picker. They can now set showInInputMethodPicker=false for
this.

Fix: 175480153
Test: manual - Test IME with showInInputMethodPicker=false isn't
 shown in the IME picker.
Test: [new tests]
 atest android.view.inputmethod.cts.InputMethodManagerTest
Test: atest CtsInputMethodTestCases
Change-Id: I18dda0525168f9195606840239507b9daba65be7